Merge branch 'jk/commit-buffer-length'

Move "commit->buffer" out of the in-core commit object and keep
track of their lengths.  Use this to optimize the code paths to
validate GPG signatures in commit objects.

* jk/commit-buffer-length:
  reuse cached commit buffer when parsing signatures
  commit: record buffer length in cache
  commit: convert commit->buffer to a slab
  commit-slab: provide a static initializer
  use get_commit_buffer everywhere
  convert logmsg_reencode to get_commit_buffer
  use get_commit_buffer to avoid duplicate code
  use get_cached_commit_buffer where appropriate
  provide helpers to access the commit buffer
  provide a helper to set the commit buffer
  provide a helper to free commit buffer
  sequencer: use logmsg_reencode in get_message
  logmsg_reencode: return const buffer
  do not create "struct commit" with xcalloc
  commit: push commit_index update into alloc_commit_node
  alloc: include any-object allocations in alloc_report
  replace dangerous uses of strbuf_attach
  commit_tree: take a pointer/len pair rather than a const strbuf

builtin/index-pack.c

@@ -786,7 +786,8 @@ static void sha1_object(const void *data, struct object_entry *obj_entry,
 			}
 			if (obj->type == OBJ_COMMIT) {
 				struct commit *commit = (struct commit *) obj;
-				commit->buffer = NULL;
+				if (detach_commit_buffer(commit, NULL) != data)
+					die("BUG: parse_object_buffer transmogrified our buffer");
 			}
 			obj->flags |= FLAG_CHECKED;
 		}