Implement support for the "blob:none" filter in git-cat-file(1), which
causes us to omit all blobs.

Signed-off-by: Patrick Steinhardt <redacted>
---
 Documentation/git-cat-file.adoc |  2 ++
 builtin/cat-file.c              | 11 ++++++++++-
 t/t1006-cat-file.sh             | 33 +++++++++++++++++++++++++++++++--
 3 files changed, 43 insertions(+), 3 deletions(-)

diff --git a/Documentation/git-cat-file.adoc b/Documentation/git-cat-file.adoc
index 7c1c888079a..c11952d9eca 100644
--- a/Documentation/git-cat-file.adoc
+++ b/Documentation/git-cat-file.adoc

@@ -86,6 +86,8 @@ OPTIONS
 	Omit objects from the list of printed objects. This can only be used in
 	combination with one of the batched modes. The '<filter-spec>' may be
 	one of the following:
++
+The form '--filter=blob:none' omits all blobs.