commit: don't rewrite shared index unnecessarilydt/commit-preserve-base-index-upon-opportunistic-cache-tree-update

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>

1 files changed, 10 insertions, 0 deletions

diff --git a/t/t0090-cache-tree.sh b/t/t0090-cache-tree.sh
index 601d02d71f..f92dd1f1dc 100755
--- a/t/t0090-cache-tree.sh
+++ b/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