commit: fix a segfault when displaying a commit with unreachable parents

I was running git show on various commits found by fsck-objects
when I found this bug.  Since find_unique_abbrev() cannot find
an abbreviation for an object not in the database, it will
return NULL, which is bad to run strlen() on.  So instead, we'll
just display the unabbreviated sha1 that we referenced in the
commit.

I'm not sure that this is the best 'fix' for it because the
commit I was trying to show was broken, but I don't think a
program should segfault even if the user tries to do something
stupid.

Signed-off-by: Eric Wong <normalperson@yhbt.net>
Signed-off-by: Junio C Hamano <junkio@cox.net>