diffcore-rename: treat a rename_limit of 0 as unlimited

In commit 89973554b5 (diffcore-rename: make diff-tree -l0 mean
-l<large>, 2017-11-29), -l0 was given a special magical "large" value,
but one which was not large enough for some uses (as can be seen from
commit 9f7e4bfa3b (diff: remove silent clamp of renameLimit,
2017-11-13).  Make 0 (or a negative value) be treated as unlimited
instead and update the documentation to mention this.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/diff-options.txt

@@ -598,6 +598,7 @@ of a delete/create pair.
 	prevents the exhaustive portion of rename/copy detection from
 	running if the number of source/destination files involved
 	exceeds the specified number.  Defaults to diff.renameLimit.
+	Note that a value of 0 is treated as unlimited.
 
 ifndef::git-format-patch[]
 --diff-filter=[(A|C|D|M|R|T|U|X|B)...[*]]::

diffcore-rename.c

@@ -1021,7 +1021,7 @@ static int too_many_rename_candidates(int num_destinations, int num_sources,
 	 * memory for the matrix anyway.
 	 */
 	if (rename_limit <= 0)
-		rename_limit = 32767;
+		return 0; /* treat as unlimited */
 	if (st_mult(num_destinations, num_sources)
 	    <= st_mult(rename_limit, rename_limit))
 		return 0;