read-tree: integrate with sparse index

Enable use of sparse index in 'git read-tree'. The integration in this patch
is limited only to usage of 'read-tree' that does not need additional
functional changes for the sparse index to behave as expected (i.e., produce
the same user-facing results as a non-sparse index sparse-checkout). To
ensure no unexpected behavior occurs, the index is explicitly expanded when:

* '--no-sparse-checkout' is specified (because it disables sparse-checkout)
* '--prefix' is specified (if the prefix is inside a sparse directory, the
  prefixed tree cannot be properly traversed)
* two or more <tree-ish> arguments are specified ('twoway_merge' and
  'threeway_merge' do not yet support merging sparse directories)

Signed-off-by: Victoria Dye <vdye@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 11 insertions, 0 deletions

diff --git a/t/t1092-sparse-checkout-compatibility.sh b/t/t1092-sparse-checkout-compatibility.sh
index 9bb5aeb979..86241b01a5 100755
--- a/t/t1092-sparse-checkout-compatibility.sh
+++ b/t/t1092-sparse-checkout-compatibility.sh
@@ -1409,6 +1409,17 @@ test_expect_success 'sparse index is not expanded: fetch/pull' '
 	ensure_not_expanded pull full base
 '
 
+test_expect_success 'sparse index is not expanded: read-tree' '
+	init_repos &&
+
+	ensure_not_expanded checkout -b test-branch update-folder1 &&
+	for MERGE_TREES in "update-folder2"
+	do
+		ensure_not_expanded read-tree -mu $MERGE_TREES &&
+		ensure_not_expanded reset --hard || return 1
+	done
+'
+
 test_expect_success 'ls-files' '
 	init_repos &&