blame: -C -C -C

When you do this, existing "blame -C -C" would not find that the
latter half of the file2 came from the existing file1:

	... both file1 and file2 are tracked ...
	$ cat file1 >>file2
	$ git add file1 file2
	$ git commit

This is because we avoid the expensive find-copies-harder code
that makes unchanged file (in this case, file1) as a candidate
for copy & paste source when annotating an existing file
(file2).  The third -C now allows it.  However, this obviously
makes the process very expensive.  We've actually seen this
patch before, but I dismissed it because it covers such a narrow
(and arguably stupid) corner case.
Signed-off-by: NJunio C Hamano <junkio@cox.net>

builtin-blame.c

@@ -55,6 +55,7 @@ static int num_commits;
 #define PICKAXE_BLAME_MOVE		01
 #define PICKAXE_BLAME_COPY		02
 #define PICKAXE_BLAME_COPY_HARDER	04
+#define PICKAXE_BLAME_COPY_HARDEST	010
 
 /*
  * blame for a blame_entry with score lower than these thresholds
@@ -1079,8 +1080,9 @@ static int find_copy_in_parent(struct scoreboard *sb,
 	 * and this code needs to be after diff_setup_done(), which
 	 * usually makes find-copies-harder imply copy detection.
 	 */
-	if ((opt & PICKAXE_BLAME_COPY_HARDER) &&
-	    (!porigin || strcmp(target->path, porigin->path)))
+	if ((opt & PICKAXE_BLAME_COPY_HARDEST)
+	    || ((opt & PICKAXE_BLAME_COPY_HARDER)
+		&& (!porigin || strcmp(target->path, porigin->path))))
 		diff_opts.find_copies_harder = 1;
 
 	if (is_null_sha1(target->commit->object.sha1))
@@ -2127,6 +2129,15 @@ int cmd_blame(int argc, const char **argv, const char *prefix)
 			blame_move_score = parse_score(arg+2);
 		}
 		else if (!prefixcmp(arg, "-C")) {
+			/*
+			 * -C enables copy from removed files;
+			 * -C -C enables copy from existing files, but only
+			 *       when blaming a new file;
+			 * -C -C -C enables copy from existing files for
+			 *          everybody
+			 */
+			if (opt & PICKAXE_BLAME_COPY_HARDER)
+				opt |= PICKAXE_BLAME_COPY_HARDEST;
 			if (opt & PICKAXE_BLAME_COPY)
 				opt |= PICKAXE_BLAME_COPY_HARDER;
 			opt |= PICKAXE_BLAME_COPY | PICKAXE_BLAME_MOVE;