Catch empty pathnames in trees during fsck

Catch empty pathnames in trees during fsck

Released versions of fast-import have been able to create a tree that
contains files or subtrees that contain no name.  Unfortunately these
trees aren't valid, but people may have actually tried to create them
due to bugs in import-tars.perl or their own fast-import frontend.

We now look for this unusual condition and warn the user if at
least one of their tree objects contains the problem.

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