reset: make --mixed sparse-aware

Remove the `ensure_full_index` guard on `read_from_tree` and update `git
reset --mixed` to ensure it can use sparse directory index entries wherever
possible. Sparse directory entries are reset using `diff_tree_oid`, which
requires `change` and `add_remove` functions to process the internal
contents of the sparse directory. The `recursive` diff option handles cases
in which `reset --mixed` must diff/merge files that are nested multiple
levels deep in a sparse directory.

The use of pathspecs with `git reset --mixed` introduces scenarios in which
internal contents of sparse directories may be matched by the pathspec. In
order to reset *all* files in the repo that may match the pathspec, the
following conditions on the pathspec require index expansion before
performing the reset:

* "magic" pathspecs
* wildcard pathspecs that do not match only in-cone files or entire sparse
  directories
* literal pathspecs matching something outside the sparse checkout
  definition

Helped-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Victoria Dye <vdye@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 17 insertions, 0 deletions

diff --git a/t/t1092-sparse-checkout-compatibility.sh b/t/t1092-sparse-checkout-compatibility.sh
index 871cc3fcb8..77e302a0ef 100755
--- a/t/t1092-sparse-checkout-compatibility.sh
+++ b/t/t1092-sparse-checkout-compatibility.sh
@@ -795,11 +795,28 @@ test_expect_success 'sparse-index is not expanded' '
 		ensure_not_expanded reset --hard $ref || return 1
 	done &&
 
+	ensure_not_expanded reset --mixed base &&
 	ensure_not_expanded reset --hard update-deep &&
 	ensure_not_expanded reset --keep base &&
 	ensure_not_expanded reset --merge update-deep &&
 	ensure_not_expanded reset --hard &&
 
+	ensure_not_expanded reset base -- deep/a &&
+	ensure_not_expanded reset base -- nonexistent-file &&
+	ensure_not_expanded reset deepest -- deep &&
+
+	# Although folder1 is outside the sparse definition, it exists as a
+	# directory entry in the index, so the pathspec will not force the
+	# index to be expanded.
+	ensure_not_expanded reset deepest -- folder1 &&
+	ensure_not_expanded reset deepest -- folder1/ &&
+
+	# Wildcard identifies only in-cone files, no index expansion
+	ensure_not_expanded reset deepest -- deep/\* &&
+
+	# Wildcard identifies only full sparse directories, no index expansion
+	ensure_not_expanded reset deepest -- folder\* &&
+
 	ensure_not_expanded checkout -f update-deep &&
 	test_config -C sparse-index pull.twohead ort &&
 	(