Add 'df_name_compare()' helper function

Add 'df_name_compare()' helper function

This new helper is identical to base_name_compare(), except it compares
conflicting directory/file entries as equal in order to help handling DF
conflicts (thus the name).

Note that while a directory name compares as equal to a regular file
with the new helper, they then individually compare _differently_ to a
filename that has a dot after the basename (because '\0' < '.' < '/').

So a directory called "foo/" will compare equal to a file "foo", even
though "foo.c" will compare after "foo" and before "foo/"

This will be used by routines that want to traverse the git namespace
but then handle conflicting entries together when possible.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>