config: teach repo_config to allow `repo` to be NULL

config: teach repo_config to allow `repo` to be NULL

The `repo` value can be NULL if a builtin command is run outside
any repository. The current implementation of `repo_config()` will
fail if `repo` is NULL.

If the `repo` is NULL the `repo_config()` can ignore the repository
configuration but it should read the other configuration sources like
the system-side configuration instead of failing.

Teach the `repo_config()` to allow `repo` to be NULL by calling the
`read_very_early_config()` which read config but only enumerate system
and global settings.

This will be useful in the following commits.

Suggested-by: Junio C Hamano <gitster@pobox.com>
Mentored-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Usman Akinyemi <usmanakinyemi202@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>