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, 38 insertions, 0 deletions

diff --git a/doc/git-union-merge.mdwn b/doc/git-union-merge.mdwn
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+# NAME
+
+git-union-merge - Join branches together using a union merge
+
+# SYNOPSIS
+
+git union-merge branch ref ref
+
+# DESCRIPTION
+
+Does a union merge between two refs, storing the result in the
+specified branch.
+
+The union merge will always succeed, but assumes that files can be merged
+simply by concacenating together lines from all the oldrefs, in any order.
+So, this is useful only for branches containing log-type data.
+
+That this does not touch the checked out working copy. It operates
+entirely on git refs and branches.
+
+# EXAMPLE
+
+	git union-merge git-annex git-annex origin/git-annex
+
+Merges the current git-annex branch, and a version from origin,
+storing the result in the git-annex branch.
+
+# BUGS
+
+File modes are not currently merged.
+
+# AUTHOR
+
+Joey Hess <joey@kitenet.net>
+
+<http://git-annex.branchable.com/>
+
+Warning: this page is automatically made into a man page via [mdwn2man](http://git.ikiwiki.info/?p=ikiwiki;a=blob;f=mdwn2man;hb=HEAD).  Edit with care