Implement support for the "object:type=" filter in git-cat-file(1),
which causes us to omit all objects that don't match the provided object
type.

Signed-off-by: Patrick Steinhardt <redacted>
---
 Documentation/git-cat-file.adoc | 3 +++
 builtin/cat-file.c              | 8 +++++++-
 t/t1006-cat-file.sh             | 6 +++++-
 3 files changed, 15 insertions(+), 2 deletions(-)

diff --git a/Documentation/git-cat-file.adoc b/Documentation/git-cat-file.adoc
index 8c474418b52..540d9dffdf9 100644
--- a/Documentation/git-cat-file.adoc
+++ b/Documentation/git-cat-file.adoc

@@ -93,6 +93,9 @@ The form '--filter=blob:limit=<n>[kmg]' omits blobs of size at least n
 bytes or units.  n may be zero.  The suffixes k, m, and g can be used
 to name units in KiB, MiB, or GiB.  For example, 'blob:limit=1k'
 is the same as 'blob:limit=1024'.
++
+The form '--filter=object:type=(tag|commit|tree|blob)' omits all objects
+which are not of the requested type.
 
 --path=<path>::
 	For use with `--textconv` or `--filters`, to allow specifying an object

diff --git a/builtin/cat-file.c b/builtin/cat-file.c
index f57bf65cb03..b374c2bb104 100644
--- a/builtin/cat-file.c
+++ b/builtin/cat-file.c

@@ -474,7 +474,8 @@ static void batch_object_write(const char *obj_name,
 
 		if (use_mailmap ||
 		    opt->objects_filter.choice == LOFC_BLOB_NONE ||
-		    opt->objects_filter.choice == LOFC_BLOB_LIMIT)
+		    opt->objects_filter.choice == LOFC_BLOB_LIMIT ||
+		    opt->objects_filter.choice == LOFC_OBJECT_TYPE)
 			data->info.typep = &data->type;
 		if (opt->objects_filter.choice == LOFC_BLOB_LIMIT)
 			data->info.sizep = &data->size;

@@ -505,6 +506,10 @@ static void batch_object_write(const char *obj_name,
 			    data->size >= opt->objects_filter.blob_limit_value)
 				return;
 			break;
+		case LOFC_OBJECT_TYPE:
+			if (data->type != opt->objects_filter.object_type)
+				return;
+			break;
 		default:
 			BUG("unsupported objects filter");