ls-tree: further tweaks of the rewrite

ls-tree: further tweaks of the rewrite

It modifies the selection a bit, so that a pathspec that is a superset of
a particular tree path will always cause it to recurse into that tree.

As an example, let's say that we do

	git-ls-tree HEAD drivers/char

_without_ the "-r". What will happen is that it will start out doing all
the base tree, and for "drivers" it will notice that it's a proper subset
of "drivers/char", so it will always recurse into _that_ tree (but not
into other trees).

Then, it will not match anything else than "char" in that subdirectory,
and because that's not a proper superset (it's an exact match), it will
_not_ recurse into it, so you get:

	[torvalds@g5 linux]$ ~/git/git-ls-tree HEAD drivers/char
	040000 tree 9568cda453aae205bb58983747fa73b9696d9d51    drivers/char

which is what you got with the old git-ls-tree too.

But interestingly, if you add the slash, it will become a proper superset
and it will recurse into _that_ subdirectory (but no deeper: so if you
want all subdirectories _below_ drivers/char/, you still need to give
"-r"):

	[torvalds@g5 linux]$ ~/git/git-ls-tree HEAD drivers/char/
	100644 blob 2b6b1d772ed776fff87927fc34adc2e40500218e    drivers/char/.gitignore
	100644 blob 56b8a2e76ab10a5c21787cb7068a846075cbaffd    drivers/char/ChangeLog
	100644 blob 970f70d498f4c814e1cf3362e33d7e23ac53c299    drivers/char/Kconfig
	...

See? This is on top of the previous two diffs, holler if you want a whole
new "everything combined" version..

It hasn't gotten lots of testing, but it should work.

		Linus