hook: allow out-of-repo 'git hook' invocations

hook: allow out-of-repo 'git hook' invocations

Since hooks can now be supplied via the config, and a config can be
present without a gitdir via the global and system configs, we can start
to allow 'git hook run' to occur without a gitdir. This enables us to do
things like run sendemail-validate hooks when running 'git send-email'
from a nongit directory.

It still doesn't make sense to look for hooks in the hookdir in nongit
repos, though, as there is no hookdir.

Signed-off-by: Emily Shaffer <emilyshaffer@google.com>
Signed-off-by: Adrian Ratiu <adrian.ratiu@collabora.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>