fully support core.symlinks=false in all relevant symlink handling code

Refactored annex link code into nice clean new library.

Audited and dealt with calls to createSymbolicLink.
Remaining calls are all safe, because:

Annex/Link.hs:  ( liftIO $ createSymbolicLink linktarget file
  only when core.symlinks=true
Assistant/WebApp/Configurators/Local.hs:                createSymbolicLink link link
  test if symlinks can be made
Command/Fix.hs: liftIO $ createSymbolicLink link file
  command only works in indirect mode
Command/FromKey.hs:     liftIO $ createSymbolicLink link file
  command only works in indirect mode
Command/Indirect.hs:                    liftIO $ createSymbolicLink l f
  refuses to run if core.symlinks=false
Init.hs:                createSymbolicLink f f2
  test if symlinks can be made
Remote/Directory.hs:    go [file] = catchBoolIO $ createSymbolicLink file f >> return True
  fast key linking; catches failure to make symlink and falls back to copy
Remote/Git.hs:          liftIO $ catchBoolIO $ createSymbolicLink loc file >> return True
  ditto
Upgrade/V1.hs:                          liftIO $ createSymbolicLink link f
  v1 repos could not be on a filesystem w/o symlinks

Audited and dealt with calls to readSymbolicLink.
Remaining calls are all safe, because:

Annex/Link.hs:		( liftIO $ catchMaybeIO $ readSymbolicLink file
  only when core.symlinks=true
Assistant/Threads/Watcher.hs:		ifM ((==) (Just link) <$> liftIO (catchMaybeIO $ readSymbolicLink file))
  code that fixes real symlinks when inotify sees them
  It's ok to not fix psdueo-symlinks.
Assistant/Threads/Watcher.hs:		mlink <- liftIO (catchMaybeIO $ readSymbolicLink file)
  ditto
Command/Fix.hs:	stopUnless ((/=) (Just link) <$> liftIO (catchMaybeIO $ readSymbolicLink file)) $ do
  command only works in indirect mode
Upgrade/V1.hs:	getsymlink = takeFileName <$> readSymbolicLink file
  v1 repos could not be on a filesystem w/o symlinks

Audited and dealt with calls to isSymbolicLink.
(Typically used with getSymbolicLinkStatus, but that is just used because
getFileStatus is not as robust; it also works on pseudolinks.)
Remaining calls are all safe, because:

Assistant/Threads/SanityChecker.hs:                             | isSymbolicLink s -> addsymlink file ms
  only handles staging of symlinks that were somehow not staged
  (might need to be updated to support pseudolinks, but this is
  only a belt-and-suspenders check anyway, and I've never seen the code run)
Command/Add.hs:         if isSymbolicLink s || not (isRegularFile s)
  avoids adding symlinks to the annex, so not relevant
Command/Indirect.hs:                            | isSymbolicLink s -> void $ flip whenAnnexed f $
  only allowed on systems that support symlinks
Command/Indirect.hs:            whenM (liftIO $ not . isSymbolicLink <$> getSymbolicLinkStatus f) $ do
  ditto
Seek.hs:notSymlink f = liftIO $ not . isSymbolicLink <$> getSymbolicLinkStatus f
  used to find unlocked files, only relevant in indirect mode
Utility/FSEvents.hs:                    | Files.isSymbolicLink s = runhook addSymlinkHook $ Just s
Utility/FSEvents.hs:                                            | Files.isSymbolicLink s ->
Utility/INotify.hs:                             | Files.isSymbolicLink s ->
Utility/INotify.hs:                     checkfiletype Files.isSymbolicLink addSymlinkHook f
Utility/Kqueue.hs:              | Files.isSymbolicLink s = callhook addSymlinkHook (Just s) change
  all above are lower-level, not relevant

Audited and dealt with calls to isSymLink.
Remaining calls are all safe, because:

Annex/Direct.hs:			| isSymLink (getmode item) =
  This is looking at git diff-tree objects, not files on disk
Command/Unused.hs:		| isSymLink (LsTree.mode l) = do
  This is looking at git ls-tree, not file on disk
Utility/FileMode.hs:isSymLink :: FileMode -> Bool
Utility/FileMode.hs:isSymLink = checkMode symbolicLinkMode
  low-level

Done!!

2 files changed, 5 insertions, 23 deletions

diff --git a/Assistant/Threads/Committer.hs b/Assistant/Threads/Committer.hs
index ba8d595f6..b40b8ad28 100644
--- a/Assistant/Threads/Committer.hs
+++ b/Assistant/Threads/Committer.hs
@@ -15,16 +15,13 @@ import Assistant.Types.Changes
 import Assistant.Commits
 import Assistant.Alert
 import Assistant.DaemonStatus
-import Assistant.Threads.Watcher
 import Assistant.TransferQueue
 import Logs.Transfer
 import Logs.Location
 import qualified Annex.Queue
 import qualified Git.Command
-import qualified Git.HashObject
 import qualified Git.LsFiles
 import qualified Git.Version
-import Git.Types
 import qualified Command.Add
 import Utility.ThreadScheduler
 import qualified Utility.Lsof as Lsof
@@ -33,6 +30,7 @@ import Types.KeySource
 import Config
 import Annex.Exception
 import Annex.Content
+import Annex.Link
 import qualified Annex
 
 import Data.Time.Clock
@@ -216,9 +214,7 @@ handleAdds delayadd cs = returnWhen (null incomplete) $ do
 				, Command.Add.link file key True
 				)
 			whenM (pure DirWatcher.eventsCoalesce <||> isDirect) $ do
-				sha <- inRepo $
-					Git.HashObject.hashObject BlobObject link
-				stageSymlink file sha
+				stageSymlink file =<< hashSymlink link
 				showEndOk
 		queueTransfers Next key (Just file) Upload
 		return $ Just change
diff --git a/Assistant/Threads/Watcher.hs b/Assistant/Threads/Watcher.hs
index 38116cdc6..8c77fa64a 100644
--- a/Assistant/Threads/Watcher.hs
+++ b/Assistant/Threads/Watcher.hs
@@ -12,7 +12,6 @@ module Assistant.Threads.Watcher (
 	WatcherException(..),
 	checkCanWatch,
 	needLsof,
-	stageSymlink,
 	onAddSymlink,
 	runHandler,
 ) where
@@ -32,13 +31,13 @@ import qualified Annex
 import qualified Annex.Queue
 import qualified Git
 import qualified Git.UpdateIndex
-import qualified Git.HashObject
 import qualified Git.LsFiles as LsFiles
 import qualified Backend
 import Annex.Content
 import Annex.Direct
 import Annex.Content.Direct
 import Annex.CatFile
+import Annex.Link
 import Git.Types
 import Config
 import Utility.ThreadScheduler
@@ -206,7 +205,7 @@ onAddSymlink file filestatus = go =<< liftAnnex (Backend.lookupFile file)
 				ensurestaged (Just link) s
 			, do
 				liftIO $ removeFile file
-				liftIO $ createSymbolicLink link file
+				liftAnnex $ Backend.makeAnnexLink link file
 				checkcontent key =<< getDaemonStatus
 				addlink link
 			)
@@ -242,10 +241,7 @@ onAddSymlink file filestatus = go =<< liftAnnex (Backend.lookupFile file)
 				Just (currlink, sha)
 					| s2w8 link == L.unpack currlink ->
 						stageSymlink file sha
-				_ -> do
-					sha <- inRepo $
-						Git.HashObject.hashObject BlobObject link
-					stageSymlink file sha
+				_ -> stageSymlink file =<< hashSymlink link
 		madeChange file LinkChange
 
 	{- When a new link appears, or a link is changed, after the startup
@@ -289,13 +285,3 @@ onErr msg _ = do
 	liftAnnex $ warning msg
 	void $ addAlert $ warningAlert "watcher" msg
 	noChange
-
-{- Adds a symlink to the index, without ever accessing the actual symlink
- - on disk. This avoids a race if git add is used, where the symlink is
- - changed to something else immediately after creation. It also allows
- - direct mode to work.
- -}
-stageSymlink :: FilePath -> Sha -> Annex ()
-stageSymlink file sha =
-	Annex.Queue.addUpdateIndex =<<
-		inRepo (Git.UpdateIndex.stageSymlink file sha)