completion: add tests showing subar checkout --detach logic

When completing words for git switch, the completion function correctly
disables the DWIM remote branch names when in the '--detach' mode. These
DWIM remote branch names will not work when the --detach option is
specified, so it does not make sense to complete them.

git checkout, however, does not disable the completion of DWIM remote
branch names in this case.

Add test cases for both git switch and git checkout showing the expected
behavior.

Signed-off-by: Jacob Keller <jacob.keller@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t9902-completion.sh

@@ -1368,6 +1368,52 @@ test_expect_success 'git checkout - a later --no-guess overrides previous --gues
 	EOF
 '
 
+test_expect_success 'git switch - with --detach, complete all references' '
+	test_completion "git switch --detach " <<-\EOF
+	HEAD Z
+	master Z
+	matching-branch Z
+	matching-tag Z
+	other/branch-in-other Z
+	other/master-in-other Z
+	EOF
+'
+
+#TODO: checkout --detach incorrectly includes DWIM remote branch names
+test_expect_failure 'git checkout - with --detach, complete only references' '
+	test_completion "git checkout --detach " <<-\EOF
+	HEAD Z
+	master Z
+	matching-branch Z
+	matching-tag Z
+	other/branch-in-other Z
+	other/master-in-other Z
+	EOF
+'
+
+test_expect_success 'git switch - with -d, complete all references' '
+	test_completion "git switch -d " <<-\EOF
+	HEAD Z
+	master Z
+	matching-branch Z
+	matching-tag Z
+	other/branch-in-other Z
+	other/master-in-other Z
+	EOF
+'
+
+#TODO: checkout -d incorrectly includes DWIM remote branch names
+test_expect_failure 'git checkout - with -d, complete only references' '
+	test_completion "git checkout -d " <<-\EOF
+	HEAD Z
+	master Z
+	matching-branch Z
+	matching-tag Z
+	other/branch-in-other Z
+	other/master-in-other Z
+	EOF
+'
+
 test_expect_success 'teardown after ref completion' '
 	git branch -d matching-branch &&
 	git tag -d matching-tag &&