config: don't implicitly use gitdir or commondir

config: don't implicitly use gitdir or commondir

'git_config_with_options()' takes a 'config_options' struct which
contains feilds for 'git_dir' and 'commondir'.  If those feilds happen
to be NULL the config machinery falls back to querying global repository
state.  Let's change this and instead use these fields in the
'config_options' struct explicilty all the time.  Since the API is
slightly changing to require these two fields to be set if callers want
the config machinery to load the repository's config, let's change the
name to 'config_with_optison()'.  This allows the config machinery to
not implicitly rely on any global repository state.

Signed-off-by: Brandon Williams <bmwill@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>