diffcore-rename: avoid unnecessary strdup'ing in break_idx

The keys of break_idx are strings from the diff_filepairs of
diff_queued_diff.  break_idx is only used in location_rename_dst(), and
that usage is always before any free'ing of the pairs (and thus the
strings in the pairs).  As such, there is no need to strdup these keys;
we can just reuse the existing strings as-is.

The merge logic doesn't make use of break detection, so this does not
affect the performance of any of my testcases.  It was just a minor
unrelated optimization noted in passing while looking at the code.

Signed-off-by: Elijah Newren <newren@gmail.com>
Reviewed-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diffcore-rename.c

@@ -54,7 +54,7 @@ static void register_rename_src(struct diff_filepair *p)
 	if (p->broken_pair) {
 		if (!break_idx) {
 			break_idx = xmalloc(sizeof(*break_idx));
-			strintmap_init(break_idx, -1);
+			strintmap_init_with_options(break_idx, -1, NULL, 0);
 		}
 		strintmap_set(break_idx, p->one->path, rename_dst_nr);
 	}