copy vs rename detection: avoid unnecessary O(n*m) loops

The core rename detection had some rather stupid code to check if a
pathname was used by a later modification or rename, which basically
walked the whole pathname space for all renames for each rename, in
order to tell whether it was a pure rename (no remaining users) or
should be considered a copy (other users of the source file remaining).

That's really silly, since we can just keep a count of users around, and
replace all those complex and expensive loops with just testing that
simple counter (but this all depends on the previous commit that shared
the diff_filespec data structure by using a separate reference count).

Note that the reference count is not the same as the rename count: they
behave otherwise rather similarly, but the reference count is tied to
the allocation (and decremented at de-allocation, so that when it turns
zero we can get rid of the memory), while the rename count is tied to
the renames and is decremented when we find a rename (so that when it
turns zero we know that it was a rename, not a copy).

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diffcore.h

@@ -31,6 +31,7 @@ struct diff_filespec {
 	unsigned long size;
 	int count;               /* Reference count */
 	int xfrm_flags;		 /* for use by the xfrm */
+	int rename_used;         /* Count of rename users */
 	unsigned short mode;	 /* file mode */
 	unsigned sha1_valid : 1; /* if true, use sha1 and trust mode;
 				  * if false, use the name and read from
@@ -58,7 +59,6 @@ struct diff_filepair {
 	struct diff_filespec *two;
 	unsigned short int score;
 	char status; /* M C R N D U (see Documentation/diff-format.txt) */
-	unsigned source_stays : 1; /* all of R/C are copies */
 	unsigned broken_pair : 1;
 	unsigned renamed_pair : 1;
 	unsigned is_unmerged : 1;