rename: Break filepairs with different types.

When we consider if a path has been totally rewritten, we did not
touch changes from symlinks to files or vice versa.  But a change
that modifies even the type of a blob surely should count as a
complete rewrite.

While we are at it, modernise diffcore-break to be aware of gitlinks (we
do not want to touch them).

Signed-off-by: Junio C Hamano <gitster@pobox.com>

cache.h

@@ -192,6 +192,13 @@ enum object_type {
 	OBJ_MAX,
 };
 
+static inline enum object_type object_type(unsigned int mode)
+{
+	return S_ISDIR(mode) ? OBJ_TREE :
+		S_ISGITLINK(mode) ? OBJ_COMMIT :
+		OBJ_BLOB;
+}
+
 #define GIT_DIR_ENVIRONMENT "GIT_DIR"
 #define GIT_WORK_TREE_ENVIRONMENT "GIT_WORK_TREE"
 #define DEFAULT_GIT_DIR_ENVIRONMENT ".git"