commit: don't rewrite shared index unnecessarily

Remove a cache invalidation which would cause the shared index to be
rewritten on as-is commits.

When the cache-tree has changed, we need to update it.  But we don't
necessarily need to update the shared index.  So setting
active_cache_changed to SOMETHING_CHANGED is unnecessary.  Instead, we
let update_main_cache_tree just update the CACHE_TREE_CHANGED bit.

In order to test this, make test-dump-split-index not segfault on
missing replace_bitmap/delete_bitmap.  This new codepath is not called
now that the test passes, but is necessary to avoid a segfault when the
new test is run with the old builtin/commit.c code.

Signed-off-by: David Turner <dturner@twopensource.com>
Acked-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/commit.c

@@ -404,10 +404,8 @@ static const char *prepare_index(int argc, const char **argv, const char *prefix
 		hold_locked_index(&index_lock, 1);
 		refresh_cache_or_die(refresh_flags);
 		if (active_cache_changed
-		    || !cache_tree_fully_valid(active_cache_tree)) {
+		    || !cache_tree_fully_valid(active_cache_tree))
 			update_main_cache_tree(WRITE_TREE_SILENT);
-			active_cache_changed = 1;
-		}
 		if (active_cache_changed) {
 			if (write_locked_index(&the_index, &index_lock,
 					       COMMIT_LOCK))

t/t0090-cache-tree.sh

@@ -218,4 +218,14 @@ test_expect_success 'no phantom error when switching trees' '
 	! test -s errors
 '
 
+test_expect_success 'switching trees does not invalidate shared index' '
+	git update-index --split-index &&
+	>split &&
+	git add split &&
+	test-dump-split-index .git/index | grep -v ^own >before &&
+	git commit -m "as-is" &&
+	test-dump-split-index .git/index | grep -v ^own >after &&
+	test_cmp before after
+'
+
 test_done

test-dump-split-index.c

@@ -26,9 +26,11 @@ int main(int ac, char **av)
 		       sha1_to_hex(ce->sha1), ce_stage(ce), ce->name);
 	}
 	printf("replacements:");
-	ewah_each_bit(si->replace_bitmap, show_bit, NULL);
+	if (si->replace_bitmap)
+		ewah_each_bit(si->replace_bitmap, show_bit, NULL);
 	printf("\ndeletions:");
-	ewah_each_bit(si->delete_bitmap, show_bit, NULL);
+	if (si->delete_bitmap)
+		ewah_each_bit(si->delete_bitmap, show_bit, NULL);
 	printf("\n");
 	return 0;
 }