branch: accept multiple upstream branches for tracking
Add a new static variant of install_branch_config() that accepts
multiple remote branch names for tracking. This will be used in an
upcoming commit that enables inheriting the tracking configuration from
a parent branch.
Currently, all callers of install_branch_config() pass only a single
remote. Make install_branch_config() a small wrapper around
install_branch_config_multiple_remotes() so that existing callers do not
need to be changed.
Signed-off-by: Josh Steadmon <steadmon@google.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>