remote: simplify url/pushurl selection

When we want to know the push urls for a remote, there is some simple
logic:

- if the user configured any remote.*.pushurl keys, then those make
the complete set of push urls

- otherwise we push to all urls in remote.*.url

Many spots implement this with a level of indirection, assigning to a
local url/url_nr pair. But since both arrays are now strvecs, we can
just use a pointer to select the appropriate strvec, shortening the code
a bit.

Even though this is now a one-liner, since it is application logic that
is present in so many places, it's worth abstracting a helper function.
In fact, we already have such a function, but it's local to
builtin/push.c. So we'll just make it available everywhere via remote.h.

There are two spots to pay special attention to here:

1. in builtin/remote.c's get_url(), we are selecting first based on
push_mode and then falling back to "url" when we're in push_mode
but no pushurl is defined. The updated code makes that much more
clear, compared to the original which had an "else" fall-through.

2. likewise in that file's set_url(), we _only_ respect push_mode,
sine the point is that we are adding to pushurl in that case
(whether it is empty or not). And thus it does not use our helper
function.

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