completion: use "git -C $there" instead of (cd $there && git ...)

We have had "git -C $there" to first go to a different directory
and run a Git command without changing the arguments for quite some
time.  Use it instead of (cd $there && git ...) in the completion
script.

This allows us to lose the work-around for misfeatures of modern
interactive-minded shells that make "cd" unusable in scripts (e.g.
end users' $CDPATH taking us to unexpected places in any POSIX
shell, and chpwd functions spewing unwanted output in zsh).

Based on Øystein Walle's idea, which was raised during the
discussion on the solution by Brandon Turner for a problem zsh users
had with RVM which mucks with chpwd_functions in users' environments
(https://github.com/wayneeseguin/rvm/issues/3076).

As $root variable, which is used to direct where to chdir to, is set
to "." based on if $2 to __git_index_files is set (not if it is empty),
the only caller of the function is fixed not to pass the optional $2
when it does not want us to switch to a different directory.  Otherwise
we would end up doing "git -C '' command...", which would not work.

Maybe we would want "git -C '' command..." to mean "do not chdir
anywhere", but that is a spearate topic.

Signed-off-by: Junio C Hamano <gitster@pobox.com>