completion: silence pseudoref existence check

In 44dbb3b (completion: support pseudoref existence checks for
reftables, 2023-12-19), we have extended the Bash completion script to
support future ref backends better by using git-rev-parse(1) to check
for pseudo-ref existence. This conversion has introduced a bug, because
even though we pass `--quiet` to git-rev-parse(1) it would still output
the resolved object ID of the ref in question if it exists.

Fix this by redirecting its stdout to `/dev/null` and add a test that
catches this behaviour. Note that the test passes even without the fix
for the "files" backend because we parse pseudo refs via the filesystem
directly in that case. But the test will fail with the "reftable"
backend.

Helped-by: Jeff King <[email protected]>
Helped-by: Johannes Schindelin <[email protected]>
Signed-off-by: Patrick Steinhardt <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: pks-t

contrib/completion/git-completion.bash

@@ -148,7 +148,7 @@ __git_pseudoref_exists ()
 	# platforms.
 	if __git_eread "$__git_repo_path/HEAD" head; then
 		if [ "$head" == "ref: refs/heads/.invalid" ]; then
-			__git rev-parse --verify --quiet "$ref"
+			__git rev-parse --verify --quiet "$ref" >/dev/null
 			return $?
 		fi
 	fi

t/t9902-completion.sh

@@ -1933,6 +1933,14 @@ test_expect_success 'git checkout - --orphan with branch already provided comple
 	EOF
 '
 
+test_expect_success 'git restore completes modified files' '
+	test_commit A a.file &&
+	echo B >a.file &&
+	test_completion "git restore a." <<-\EOF
+	a.file
+	EOF
+'
+
 test_expect_success 'teardown after ref completion' '
 	git branch -d matching-branch &&
 	git tag -d matching-tag &&
@@ -2728,4 +2736,27 @@ test_expect_success '__git_complete' '
 	test_must_fail __git_complete ga missing
 '
 
+test_expect_success '__git_pseudoref_exists' '
+	test_when_finished "rm -rf repo" &&
+	git init repo &&
+	(
+		cd repo &&
+		sane_unset __git_repo_path &&
+
+		# HEAD points to an existing branch, so it should exist.
+		test_commit A &&
+		__git_pseudoref_exists HEAD >output 2>&1 &&
+		test_must_be_empty output &&
+
+		# CHERRY_PICK_HEAD does not exist, so the existence check should fail.
+		! __git_pseudoref_exists CHERRY_PICK_HEAD >output 2>&1 &&
+		test_must_be_empty output &&
+
+		# CHERRY_PICK_HEAD points to a commit, so it should exist.
+		git update-ref CHERRY_PICK_HEAD A &&
+		__git_pseudoref_exists CHERRY_PICK_HEAD >output 2>&1 &&
+		test_must_be_empty output
+	)
+'
+
 test_done