completion: don't guard git executions with __gitdir()

completion: don't guard git executions with __gitdir()

Three completion functions, namely __git_index_files(), __git_heads()
and __git_tags(), first run __gitdir() and check that the path it
outputs exists, i.e. that there is a git repository, and run a git
command only if there is one.

After the previous changes in this series there are no further uses of
__gitdir()'s output in these functions besides those checks.  And
those checks are unnecessary, because we can just execute those git
commands outside of a repository and let them error out.  We don't
perform such a check in other places either.

Remove this check and the __gitdir() call from these functions,
sparing the fork()+exec() overhead of the command substitution and the
potential 'git rev-parse' execution.

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