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>

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