Merge branch 'ps/cat-file-filter-batch'

"git cat-file --batch" and friends learned to allow "--filter=" to
omit certain objects, just like the transport layer does.

* ps/cat-file-filter-batch:
  builtin/cat-file: use bitmaps to efficiently filter by object type
  builtin/cat-file: deduplicate logic to iterate over all objects
  pack-bitmap: introduce function to check whether a pack is bitmapped
  pack-bitmap: add function to iterate over filtered bitmapped objects
  pack-bitmap: allow passing payloads to `show_reachable_fn()`
  builtin/cat-file: support "object:type=" objects filter
  builtin/cat-file: support "blob:limit=" objects filter
  builtin/cat-file: support "blob:none" objects filter
  builtin/cat-file: wire up an option to filter objects
  builtin/cat-file: introduce function to report object status
  builtin/cat-file: rename variable that tracks usage

Author: gitster

Documentation/git-cat-file.adoc

@@ -81,6 +81,25 @@ OPTIONS
 	end-of-line conversion, etc). In this case, `<object>` has to be of
 	the form `<tree-ish>:<path>`, or `:<path>`.
 
+--filter=<filter-spec>::
+--no-filter::
+	Omit objects from the list of printed objects. This can only be used in
+	combination with one of the batched modes. Excluded objects that have
+	been explicitly requested via any of the batch modes that read objects
+	via standard input (`--batch`, `--batch-check`) will be reported as
+	"filtered". Excluded objects in `--batch-all-objects` mode will not be
+	printed at all. The '<filter-spec>' may be one of the following:
++
+The form '--filter=blob:none' omits all blobs.
++
+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
 	name and a path separately, e.g. when it is difficult to figure out
@@ -340,6 +359,13 @@ the repository, then `cat-file` will ignore any custom format and print:
 <object> SP missing LF
 ------------
 
+If a name is specified on stdin that is filtered out via `--filter=`,
+then `cat-file` will ignore any custom format and print:
+
+------------
+<object> SP excluded LF
+------------
+
 If a name is specified that might refer to more than one object (an ambiguous short sha), then `cat-file` will ignore any custom format and print:
 
 ------------