Make 'traverse_trees()' traverse conflicting DF entries in parallel

Make 'traverse_trees()' traverse conflicting DF entries in parallel

This makes the traverse_trees() entry comparator routine use the more
relaxed form of name comparison that considers files and directories
with the same name identical.

We pass in a separate mask for just the directory entries, so that the
callback routine can decide (if it wants to) to only handle one or the
other type, but generally most (all?) users are expected to really want
to see the case of a name 'foo' showing up in one tree as a file and in
another as a directory at the same time.

In particular, moving 'unpack_trees()' over to use this tree traversal
mechanism requires this.

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