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

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>