rename detection with -M100 means "exact renames only".

When the user is interested in pure renames, there is no point
doing the similarity scores.  This changes the score argument
parsing to special case -M100 (otherwise, it is a precision
scaled value 0 <= v < 1 and would mean 0.1, not 1.0 --- if you
do mean 0.1, you can say -M1), and optimizes the diffcore_rename
transformation to only look at pure renames in that case.

Signed-off-by: Junio C Hamano <junkio@cox.net>

diff.c

@@ -853,6 +853,10 @@ static int parse_num(const char **cp_p)
 	}
 	*cp_p = cp;
 
+	/* special case: -M100 would mean 1.0 not 0.1 */
+	if (num == 100 && scale == 1000)
+		return MAX_SCORE;
+
 	/* user says num divided by scale and we say internally that
 	 * is MAX_SCORE * num / scale.
 	 */

diffcore-rename.c

@@ -307,6 +307,9 @@ void diffcore_rename(struct diff_options *options)
 	if (rename_count == rename_dst_nr)
 		goto cleanup;
 
+	if (minimum_score == MAX_SCORE)
+		goto cleanup;
+
 	num_create = (rename_dst_nr - rename_count);
 	num_src = rename_src_nr;
 	mx = xmalloc(sizeof(*mx) * num_create * num_src);