stash: prefer plumbing over git-diff

When creating a stash, we need to look at the diff between
the working tree and HEAD, and do so using the git-diff
porcelain.  Because git-diff enables porcelain config like
renames by default, this causes at least one problem. The
--name-only format will not mention the source side of a
rename, meaning we will fail to stash a deletion that is
part of a rename.

We could fix that case by passing --no-renames, but this is
a symptom of a larger problem. We should be using the
diff-index plumbing here, which does not have renames
enabled by default, and also does not respect any
potentially confusing config options.

Reported-by: Matthew Patey <matthew.patey2167@gmail.com>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-stash.sh

@@ -116,7 +116,7 @@ create_stash () {
 			git read-tree --index-output="$TMPindex" -m $i_tree &&
 			GIT_INDEX_FILE="$TMPindex" &&
 			export GIT_INDEX_FILE &&
-			git diff --name-only -z HEAD -- >"$TMP-stagenames" &&
+			git diff-index --name-only -z HEAD -- >"$TMP-stagenames" &&
 			git update-index -z --add --remove --stdin <"$TMP-stagenames" &&
 			git write-tree &&
 			rm -f "$TMPindex"

t/t3903-stash.sh

@@ -731,4 +731,13 @@ test_expect_success 'stash list --cc shows combined diff' '
 	test_cmp expect actual
 '
 
+test_expect_success 'stash is not confused by partial renames' '
+	mv file renamed &&
+	git add renamed &&
+	git stash &&
+	git stash apply &&
+	test_path_is_file renamed &&
+	test_path_is_missing file
+'
+
 test_done