Do not complain about "no common commits" in an empty repo

If the repo is empty, we know that already, thank you very much.
So shut fetch-pack up about that case.

Fixes issue 3, too.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

fetch-pack.c

@@ -289,7 +289,8 @@ done:
 		}
 		flushes--;
 	}
-	return retval;
+	/* it is no error to fetch into a completely empty repo */
+	return count ? retval : 0;
 }
 
 static struct commit_list *complete;