Git::unquote_path(): throw an exception on bad path

This is what the other routines in Git.pm do if there's an error.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

perl/Git.pm

@@ -1493,8 +1493,8 @@ when not using -z or when parsing the output of diff -u.
 					$_ = $2;
 					last;
 				}
-				# This is malformed -- just return it as-is for now.
-				return $_[0];
+				# This is malformed
+				throw Error::Simple("invalid quoted path $_[0]");
 			}
 			$_ = $remainder;
 		}