git: update to v2.0.3

This is slightly more involved than just bumping the version number
because it pulls in a change to convert the commit buffer to a slab,
removing the "buffer" field from "struct commit".  All sites that access
"commit->buffer" have been changed to use the new functions provided for
this purpose.

Signed-off-by: John Keeping <john@keeping.me.uk>

ui-stats.c

@@ -244,7 +244,7 @@ static struct string_list collect_stats(struct cgit_period *period)
 	memset(&authors, 0, sizeof(authors));
 	while ((commit = get_revision(&rev)) != NULL) {
 		add_commit(&authors, commit, period);
-		free(commit->buffer);
+		free_commit_buffer(commit);
 		free_commit_list(commit->parents);
 	}
 	return authors;