completion: ensure that the repository path given on the command line exists

The __gitdir() helper function prints the path to the git repository
to its stdout or stays silent and returns with error when it can't
find a repository or when the repository given via $GIT_DIR doesn't
exist.

This is not the case, however, when the path in $__git_dir, i.e. the
path to the repository specified on the command line via 'git
--git-dir=<path>', doesn't exist: __gitdir() still outputs it as if it
were a real existing repository, making some completion functions
believe that they operate on an existing repository.

Check that the path in $__git_dir exists and return with error without
printing anything to stdout if it doesn't.

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

contrib/completion/git-completion.bash

@@ -40,6 +40,7 @@ __gitdir ()
 {
 	if [ -z "${1-}" ]; then
 		if [ -n "${__git_dir-}" ]; then
+			test -d "$__git_dir" || return 1
 			echo "$__git_dir"
 		elif [ -n "${GIT_DIR-}" ]; then
 			test -d "${GIT_DIR-}" || return 1