ls-tree: further cleanup to parallel ls-files.

To get more a "git-ls-files" approach, this trivial patch (on top of my
previous one) enables recursion, and doesn't show partial trees.

[jc: after further discussion, this version enables recursion by default,
 and you can disable it with "-d" flag.

	git-ls-tree -d HEAD Documentation/no/such/directory

 shows Documentation tree (without -d it shows nothing).

	git-ls-tree HEAD

 shows everything from the tree.  Only to get the single level from the top

	git-ls-tree -d HEAD

 is needed.  But there is no way to get the single level with pathspec.
 You need to extract the object name of Documentation tree from the parent
 tree and run

	git-ls-tree -d $tree_id_of_Documentation_tree

 to get something similar to what you can get from the current

	git-ls-tree HEAD Documentation
 ]

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