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/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