Use branch names in 'git-rebase -m' conflict hunks.

Use branch names in 'git-rebase -m' conflict hunks.

If a three-way merge in git-rebase generates a conflict then we
should take advantage of git-merge-recursive's ability to include
the branch name of each side of the conflict hunk by setting the
GITHEAD_* environment variables.

In the case of rebase there aren't really two clear branches; we
have the branch we are rebasing onto, and we have the branch we are
currently rebasing.  Since most conflicts will be arising between
the user's current branch and the branch they are rebasing onto
we assume the stuff that isn't in the current commit is the "onto"
branch and the stuff in the current commit is the "current" branch.

This assumption may however come up wrong if the user resolves one
conflict in such a way that it conflicts again on a future commit
also being rebased.  In this case the user's prior resolution will
appear to be in the "onto" part of the hunk.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>