completion: improve handling of --detach in checkout

Just like git switch, we should not complete DWIM remote branch names
if --detach has been specified. To avoid this, refactor _git_checkout in
a similar way to _git_switch.

Note that we don't simply clear dwim_opt when we find -d or --detach, as
we will be adding other modes and checks, making this flow easier to
follow.

Update the previously failing tests to show that the breakage has been
resolved.

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

contrib/completion/git-completion.bash

@@ -1489,7 +1489,12 @@ _git_checkout ()
 		;;
 	*)
 		local dwim_opt="$(__git_checkout_default_dwim_mode)"
-		__git_complete_refs $dwim_opt
+
+		if [ -n "$(__git_find_on_cmdline "-d --detach")" ]; then
+			__git_complete_refs --mode="refs"
+		else
+			__git_complete_refs $dwim_opt --mode="refs"
+		fi
 		;;
 	esac
 }

t/t9902-completion.sh

@@ -1371,8 +1371,7 @@ test_expect_success 'git switch - with --detach, complete all references' '
 	EOF
 '
 
-#TODO: checkout --detach incorrectly includes DWIM remote branch names
-test_expect_failure 'git checkout - with --detach, complete only references' '
+test_expect_success 'git checkout - with --detach, complete only references' '
 	test_completion "git checkout --detach " <<-\EOF
 	HEAD Z
 	master Z
@@ -1394,8 +1393,7 @@ test_expect_success 'git switch - with -d, complete all references' '
 	EOF
 '
 
-#TODO: checkout -d incorrectly includes DWIM remote branch names
-test_expect_failure 'git checkout - with -d, complete only references' '
+test_expect_success 'git checkout - with -d, complete only references' '
 	test_completion "git checkout -d " <<-\EOF
 	HEAD Z
 	master Z