completion: don't list 'HEAD' when trying refs completion outside of a repo

When refs completion is attempted while not in a git repository, the
completion script offers 'HEAD' erroneously.

Check early in __git_refs() that there is either a repository or a
remote to work on, and return early if neither is given.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -346,7 +346,11 @@ __git_refs ()
 	local list_refs_from=path remote="${1-}"
 	local format refs pfx
 
-	if [ -n "$remote" ]; then
+	if [ -z "$remote" ]; then
+		if [ -z "$dir" ]; then
+			return
+		fi
+	else
 		if __git_is_configured_remote "$remote"; then
 			# configured remote takes precedence over a
 			# local directory with the same name
@@ -360,7 +364,7 @@ __git_refs ()
 		fi
 	fi
 
-	if [ "$list_refs_from" = path ] && [ -d "$dir" ]; then
+	if [ "$list_refs_from" = path ]; then
 		case "$cur" in
 		refs|refs/*)
 			format="refname"

t/t9902-completion.sh

@@ -599,7 +599,7 @@ test_expect_success '__git_refs - non-existing URL remote - full refs' '
 	test_must_be_empty "$actual"
 '
 
-test_expect_failure '__git_refs - not in a git repository' '
+test_expect_success '__git_refs - not in a git repository' '
 	(
 		GIT_CEILING_DIRECTORIES="$ROOT" &&
 		export GIT_CEILING_DIRECTORIES &&