pull: plug minor memory leak after using is_descendant_of()

cmd_pull() builds a commit_list to pass a single potential ancestor to
is_descendant_of().  The latter leaves the list intact.  Release the
allocated memory after the call.

Leaking in cmd_*() isn't a big deal, but sets a bad example for other
users of is_descendant_of().

Signed-off-by: René Scharfe <l.s.r@web.de>
Acked-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/pull.c

@@ -994,6 +994,7 @@ int cmd_pull(int argc, const char **argv, const char *prefix)
 				opt_ff = "--ff-only";
 				ret = run_merge();
 			}
+			free_commit_list(list);
 		}
 		ret = run_rebase(&curr_head, merge_heads.oid, &rebase_fork_point);