remote: drop "explicit" parameter from remote_ref_for_branch()

remote: drop "explicit" parameter from remote_ref_for_branch()

Commit 9700fae5ee (for-each-ref: let upstream/push report the remote
ref name, 2017-11-07) added a remote_ref_for_branch() helper, which
is modeled after remote_for_branch(). This includes providing an
"explicit" out-parameter that tells the caller whether the remote
was configured by the user, or whether we picked a default name like
"origin".

But unlike remote names, there is no default name when the user
didn't configure one.  The only way the "explicit" parameter is used
by the caller is to use the value returned from the helper when it
is set, and use an empty string otherwise, ignoring the returned
value from the helper.

Let's drop the "explicit" out-parameter, and return NULL when the
returned value from the helper should be ignored, to simplify the
function interface.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Damien Robert <damien.olivier.robert+git@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>