completion: fix most spots not respecting 'git --git-dir=<path>'

completion: fix most spots not respecting 'git --git-dir=<path>'

The completion script already respects the path to the repository
specified on the command line most of the time, here we add the
necessary '--git-dir=$(__gitdir)' options to most of the places where
git was executed without it.  The exceptions where said option is not
added are the git invocations:

  - in __git_refs() which are non-trivial and will be the subject of
    the following patch,

  - getting the list of git commands, merge strategies and archive
    formats, because these are independent from the repository and
    thus don't need it, and

  - the 'git rev-parse --git-dir' in __gitdir() itself.

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