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, 0 deletions

diff --git a/.gitignore b/.gitignore
index b73167c92..9a4bc80de 100644
--- a/.gitignore
+++ b/.gitignore
@@ -5,8 +5,10 @@ configure
 SysConfig.hs
 git-annex
 git-annex-shell
+git-union-merge
 git-annex.1
 git-annex-shell.1
+git-union-merge.1
 doc/.ikiwiki
 html
 *.tix