completion: respect 'git --git-dir=<path>' when listing remote refs

In __git_refs() the git commands listing refs, both short and full, from a given remote repository are run without giving them the path to the git repository which might have been specified on the command line via 'git --git-dir=<path>'. This is bad, those git commands should access the 'refs/remotes/<remote>/' hierarchy or the remote and credentials configuration in that specified repository. Use the __gitdir() helper only to find the path to the .git directory and pass the resulting path to the 'git ls-remote' and 'for-each-ref' executions that list remote refs. While modifying that 'for-each-ref' line, remove the superfluous disambiguating doubledash. Don't use __gitdir() to check that the given remote is on the file system: basically it performs only a single if statement for us at the considerable cost of fork()ing a subshell for a command substitution. We are better off to perform all the necessary checks of the remote in __git_refs(). Though __git_refs() was the last remaining callsite that passed a remote to __gitdir(), don't delete __gitdir()'s remote-handling part yet, just in case some users' custom completion scriptlets depend on it. Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>