completion: respect 'git -C <path>'

completion: respect 'git -C <path>'

'git -C <path>' option(s) on the command line should be taken into
account during completion, because

  - like '--git-dir=<path>', it can lead us to a different repository,

  - a few git commands executed in the completion script do care about
    in which directory they are executed, and

  - the command for which we are providing completion might care about
    in which directory it will be executed.

However, unlike '--git-dir=<path>', the '-C <path>' option can be
specified multiple times and their effect is cumulative, so we can't
just store a single '<path>' in a variable.  Nor can we simply
concatenate a path from '-C <path1> -C <path2> ...', because e.g. (in
an arguably pathological corner case) a relative path might be
followed by an absolute path.

Instead, store all '-C <path>' options word by word in the
$__git_C_args array in the main git completion function, and pass this
array, if present, to 'git rev-parse --absolute-git-dir' when
discovering the repository in __gitdir(), and let it take care of
multiple options, relative paths, absolute paths and everything.

Also pass all '-C <path> options via the $__git_C_args array to those
git executions which require a worktree and for which it matters from
which directory they are executed from.  There are only three such
cases:

  - 'git diff-index' and 'git ls-files' in __git_ls_files_helper()
    used for git-aware filename completion, and

  - the 'git ls-tree' used for completing the 'ref:path' notation.

The other git commands executed in the completion script don't need
these '-C <path>' options, because __gitdir() already took those
options into account.  It would not hurt them, either, but let's not
induce unnecessary code churn.

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