config: stop checking whether the_repository is NULL

config: stop checking whether the_repository is NULL

Since the previous commit, our invariant that the_repository is never
NULL is restored, and we can stop being defensive in include_by_branch().

We can confirm the fix by showing that an onbranch config include will
not cause a segfault when run outside a git repository. I've put this in
t1309-early-config since it's related to the case added by 85fe0e800c
(config: work around bug with includeif:onbranch and early config,
2019-07-31), though technically the issue was with
read_very_early_config() and not read_early_config().

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>