completion: extract repository discovery from __gitdir()

completion: extract repository discovery from __gitdir()

To prepare for caching the path to the repository in the following
commit, extract the repository discovering part of __gitdir() into the
__git_find_repo_path() helper function, which stores the found path in
the $__git_repo_path variable instead of printing it.  Make __gitdir()
a wrapper around this new function.  Declare $__git_repo_path local in
the toplevel completion functions __git_main() and __gitk_main() to
ensure that it never leaks into the environment and influences
subsequent completions (though this isn't necessary right now, as
__gitdir() is still only executed in subshells, but will matter for
the following commit).

Adjust tests checking __gitdir() or any other completion function
calling __gitdir() to perform those checks in a subshell to prevent
$__git_repo_path from leaking into the test environment.  Otherwise
leave the tests unchanged to demonstrate that this change doesn't
alter __gitdir()'s behavior.

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