wt-status.c: set commitable bit if there is a meaningful merge.

The 'commit --dry-run' and 'commit' return values differed if a
conflicted merge had been resolved and the resulting commit would
record the same tree as the parent.

Update show_merge_in_progress to set the commitable bit if conflicts
have been resolved and a merge is in progress.

Signed-off-by: Stephen P. Smith <ischis2@cox.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>