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.

doc/git-union-merge.mdwn

@@ -0,0 +1,38 @@
+# 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

doc/todo/branching.mdwn

@@ -148,10 +148,8 @@ problem generically. Something like this:
 * For remotes, there are also `origin/B`, `otherremote/B`, etc.
 * To merge two branches `B` and `foo/B`, construct a merge commit that
   makes each file have all lines that were in either version of the file,
-  with duplicates removed (probably). Do this without checking out a tree,
-  or using a temporary directory. (One easy but expensive way is to just
-  check out the branch to a temp dir, union merge into it, and remove the
-  temp dir ... but it should be possible to do it without using a temp dir.)
+  with duplicates removed (probably). Do this without checking out a tree.
+  -- now implemented as git-union-merge
 * As a `post-merge` hook, merge `*/B` into `B`. This will ensure `B`
   is always up-to-date after a pull from a remote.
 * When pushing to a remote, nothing need to be done, except ensure