pack: add git_packfile_resolve_header

To paraphrase @peff:

You can get both size and type from a packed object reasonably cheaply.
If you have:

* An object that is not a delta; both type and size are available in the
  packfile header.
* An object that is a delta. The packfile type will be OBJ_*_DELTA, and
  you have to resolve back to the base to find the real type. That means
  potentially a lot of packfile index lookups, but each one is
  relatively cheap. For the size, you inflate the first few bytes of the
  delta, whose header will tell you the resulting size of applying the
  delta to the base.

For simplicity, we just decompress the whole delta for now.

src/pack.c

@@ -277,6 +277,56 @@ int git_packfile_unpack_header(
 	return 0;
 }
 
+int git_packfile_resolve_header(
+		size_t *size_p,
+		git_otype *type_p,
+		struct git_pack_file *p,
+		git_off_t offset)
+{
+	git_mwindow *w_curs = NULL;
+	git_off_t curpos = offset;
+	size_t size;
+	git_otype type;
+	git_off_t base_offset;
+	int error;
+
+	error = git_packfile_unpack_header(&size, &type, &p->mwf, &w_curs, &curpos);
+	git_mwindow_close(&w_curs);
+	if (error < 0)
+		return error;
+
+	if (type == GIT_OBJ_OFS_DELTA || type == GIT_OBJ_REF_DELTA) {
+		size_t base_size;
+		git_rawobj delta;
+		base_offset = get_delta_base(p, &w_curs, &curpos, type, offset);
+		git_mwindow_close(&w_curs);
+		error = packfile_unpack_compressed(&delta, p, &w_curs, &curpos, size, type);
+		git_mwindow_close(&w_curs);
+		if (error < 0)
+			return error;
+		error = git__delta_read_header(delta.data, delta.len, &base_size, size_p);
+		git__free(delta.data);
+		if (error < 0)
+			return error;
+	} else
+		*size_p = size;
+
+	while (type == GIT_OBJ_OFS_DELTA || type == GIT_OBJ_REF_DELTA) {
+		curpos = base_offset;
+		error = git_packfile_unpack_header(&size, &type, &p->mwf, &w_curs, &curpos);
+		git_mwindow_close(&w_curs);
+		if (error < 0)
+			return error;
+		if (type != GIT_OBJ_OFS_DELTA && type != GIT_OBJ_REF_DELTA)
+			break;
+		base_offset = get_delta_base(p, &w_curs, &curpos, type, base_offset);
+		git_mwindow_close(&w_curs);
+	}
+	*type_p = type;
+
+	return error;
+}
+
 static int packfile_unpack_delta(
 		git_rawobj *obj,
 		struct git_pack_file *p,

src/pack.h

@@ -83,6 +83,12 @@ int git_packfile_unpack_header(
 		git_mwindow **w_curs,
 		git_off_t *curpos);
 
+int git_packfile_resolve_header(
+		size_t *size_p,
+		git_otype *type_p,
+		struct git_pack_file *p,
+		git_off_t offset);
+
 int git_packfile_unpack(git_rawobj *obj, struct git_pack_file *p, git_off_t *obj_offset);
 int packfile_unpack_compressed(
 	git_rawobj *obj,