checkout -m: recreate merge when checking out of unmerged index
This teaches git-checkout to recreate a merge out of unmerged
index entries while resolving conflicts.
With this patch, checking out an unmerged path from the index
now have the following possibilities:
* Without any option, an attempt to checkout an unmerged path
will atomically fail (i.e. no other cleanly-merged paths are
checked out either);
* With "-f", other cleanly-merged paths are checked out, and
unmerged paths are ignored;
* With "--ours" or "--theirs, the contents from the specified
stage is checked out;
* With "-m" (we should add "--merge" as synonym), the 3-way merge
is recreated from the staged object names and checked out.
Signed-off-by: Junio C Hamano <gitster@pobox.com>