completion: don't use __gitdir() for git commands

Several completion functions contain the following pattern to run git
commands respecting the path to the repository specified on the
command line:

  git --git-dir="$(__gitdir)" <cmd> <options>

This imposes the overhead of fork()ing a subshell for the command
substitution and potentially fork()+exec()ing 'git rev-parse' inside
__gitdir().

Now, if neither '--gitdir=<path>' nor '-C <path>' options are
specified on the command line, then those git commands are perfectly
capable to discover the repository on their own.  If either one or
both of those options are specified on the command line, then, again,
the git commands could discover the repository, if we pass them all of
those options from the command line.

This means we don't have to run __gitdir() at all for git commands and
can spare its fork()+exec() overhead.

Use Bash parameter expansions to check the $__git_dir variable and
$__git_C_args array and to assemble the appropriate '--git-dir=<path>'
and '-C <path>' options if either one or both are present on the
command line.  These parameter expansions are, however, rather long,
so instead of changing all git executions and make already long lines
even longer, encapsulate running git with '--git-dir=<path> -C <path>'
options into the new __git() wrapper function.  Furthermore, this
wrapper function will also enable us to silence error messages from
git commands uniformly in one place in a later commit.

There's one tricky case, though: in __git_refs() local refs are listed
with 'git for-each-ref', where "local" is not necessarily the
repository we are currently in, but it might mean a remote repository
in the filesystem (e.g. listing refs for 'git fetch /some/other/repo
<TAB>').  Use one-shot variable assignment to override $__git_dir with
the path of the repository where the refs should come from.  Although
one-shot variable assignments in front of shell functions are to be
avoided in our scripts in general, in the Bash completion script we
can do that safely.

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

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