list-objects-filter: teach tree:# how to handle >0

Implement positive values for <depth> in the tree:<depth> filter. The
exact semantics are described in Documentation/rev-list-options.txt.

The long-term goal at the end of this is to allow a partial clone to
eagerly fetch an entire directory of files by fetching a tree and
specifying <depth>=1. This, for instance, would make a build operation
fast and convenient. It is fast because the partial clone does not need
to fetch each file individually, and convenient because the user does
not need to supply a sparse-checkout specification.

Another way of considering this feature is as a way to reduce
round-trips, since the client can get any number of levels of
directories in a single request, rather than wait for each level of tree
objects to come back, whose entries are used to construct a new request.

Signed-off-by: Matthew DeVore <matvore@google.com>
Reviewed-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/rev-list-options.txt

@@ -734,8 +734,13 @@ specification contained in <path>.
 +
 The form '--filter=tree:<depth>' omits all blobs and trees whose depth
 from the root tree is >= <depth> (minimum depth if an object is located
-at multiple depths in the commits traversed). Currently, only <depth>=0
-is supported, which omits all blobs and trees.
+at multiple depths in the commits traversed). <depth>=0 will not include
+any trees or blobs unless included explicitly in the command-line (or
+standard input when --stdin is used). <depth>=1 will include only the
+tree and blobs which are referenced directly by a commit reachable from
+<commit> or an explicitly-given object. <depth>=2 is like <depth>=1
+while also including trees and blobs one more level removed from an
+explicitly-given commit or tree.
 
 --no-filter::
 	Turn off any previous `--filter=` argument.