Leave merge failures in the filesystem

This changes how we handle merges: if a automated merge
fails, we will leave the index as a clean entry pointing
to the original branch, and leave the actual file _dirty_
the way the "merge" program left it.

You can then just do "git-diff-files -p" to see what the
merge conflicts did, fix them up, and commit the end result.

NOTE NOTE NOTE! Do _not_ use "git commit" to commit such
a merge. It won't set the parents right. I'll need to fix
that. In the meantime, you'd need to merge using

	git-commit-tree $(git-write) -p HEAD -p MERGE_HEAD

or something like that by hand.