list-objects-filter: implement filter tree:0

Teach list-objects the "tree:0" filter which allows for filtering
out all tree and blob objects (unless other objects are explicitly
specified by the user). The purpose of this patch is to allow smaller
partial clones.

The name of this filter - tree:0 - does not explicitly specify that
it also filters out all blobs, but this should not cause much confusion
because blobs are not at all useful without the trees that refer to
them.

I also considered only:commits as a name, but this is inaccurate because
it suggests that annotated tags are omitted, but actually they are
included.

The name "tree:0" allows later filtering based on depth, i.e. "tree:1"
would filter out all but the root tree and blobs. In order to avoid
confusion between 0 and capital O, the documentation was worded in a
somewhat round-about way that also hints at this future improvement to
the feature.

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

1 files changed, 5 insertions, 0 deletions

diff --git a/Documentation/rev-list-options.txt b/Documentation/rev-list-options.txt
index 7b273635de..5f1672913b 100644
--- a/Documentation/rev-list-options.txt
+++ b/Documentation/rev-list-options.txt
@@ -731,6 +731,11 @@ the requested refs.
 +
 The form '--filter=sparse:path=<path>' similarly uses a sparse-checkout
 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.
 
 --no-filter::
 	Turn off any previous `--filter=` argument.