add git-union-merge

This is a new git subcommand, that does a generic union merge operation
between two refs, storing the result in a branch. It operates efficiently
without touching the working tree. It does need to write out a temporary
index file, and may need to write out some other temp files as well.

This could be useful for anything that stores data in a branch,
and needs to merge changes into that branch without actually checking the
branch out. Since conflict handling can't be done without a working copy,
the merge type is always a union merge, which is fine for data stored in
log format (as git-annex does), or in non-conflicting files
(as pristine-tar does).

This probably belongs in git proper, but it will live in git-annex for now.

---

Plan is to move .git-annex/ to a git-annex branch, and use git-union-merge
to handle merging changes when pulling from remotes.

Some preliminary benchmarking using real .git-annex/ data indicates
that it's quite fast, except for the "git add" call, which is as slow
as "git add" tends to be with a big index.

1 files changed, 2 insertions, 1 deletions

diff --git a/GitRepo.hs b/GitRepo.hs
index 24bc9b5c2..0bee2842a 100644
--- a/GitRepo.hs
+++ b/GitRepo.hs
@@ -38,6 +38,7 @@ module GitRepo (
 	gitCommandLine,
 	run,
 	pipeRead,
+	pipeNullSplit,
 	attributes,
 	remotes,
 	remotesAdd,
@@ -412,7 +413,7 @@ typeChangedFiles' repo l middle = pipeNullSplit repo $ start ++ middle ++ end
 		end = [Param "--"] ++ map File l
 
 {- Reads null terminated output of a git command (as enabled by the -z 
- - parameter), and splits it into a list of files. -}
+ - parameter), and splits it into a list of files/lines/whatever. -}
 pipeNullSplit :: Repo -> [CommandParam] -> IO [FilePath]
 pipeNullSplit repo params = do
 	fs0 <- pipeRead repo params