git_config: add support for repo-specific settings

This allows people to write ~/.repoconfig/config akin to ~/.gitconfig
and .repo/config akin to .git/config.  This allows us to add settings
specific to repo without mixing up git, and to persist in general.

Change-Id: I1c6fbe31e63fb8ce26aa85335349c6ae5b1712c6
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/255832
Reviewed-by: David Pursehouse <dpursehouse@collab.net>
Tested-by: Mike Frysinger <vapier@google.com>