Allow merging bare trees in merge-recursive.

Allow merging bare trees in merge-recursive.

To support wider use cases, such as from within `git am -3`, the
merge-recursive utility needs to accept not just commit-ish but
also tree-ish as arguments on its command line.

If given a tree-ish then merge-recursive will create a virtual commit
wrapping it, with the subject of the commit set to the best name we
can derive for that tree, which is either the command line string
(probably the SHA1), or whatever string appears in GITHEAD_*.

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