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>