completion: improve existence check for pseudo-refs
Improve the existence check along the following lines:
- Stop stripping the "ref :" prefix and compare to the expected value
directly. This allows us to drop a now-unused variable that was
previously leaking into the user's shell.
- Mark the "head" variable as local so that we don't leak its value
into the user's shell.
- Stop manually handling the `-C $__git_repo_path` option, which the
`__git ()` wrapper aleady does for us.
- In simlar spirit, stop redirecting stderr, which is also handled by
the wrapper already.
Suggested-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>