remote.c: provide per-branch pushremote name

remote.c: provide per-branch pushremote name

When remote.c loads its config, it records the
branch.*.pushremote for the current branch along with the
global remote.pushDefault value, and then binds them into a
single value: the default push for the current branch. We
then pass this value (which may be NULL) to remote_get_1
when looking up a remote for push.

This has a few downsides:

  1. It's confusing. The early-binding of the "current
     value" led to bugs like the one fixed by 98b406f
     (remote: handle pushremote config in any order,
     2014-02-24). And the fact that pushremotes fall back to
     ordinary remotes is not explicit at all; it happens
     because remote_get_1 cannot tell the difference between
     "we are not asking for the push remote" and "there is
     no push remote configured".

  2. It throws away intermediate data. After read_config()
     finishes, we have no idea what the value of
     remote.pushDefault was, because the string has been
     overwritten by the current branch's
     branch.*.pushremote.

  3. It doesn't record other data. We don't note the
     branch.*.pushremote value for anything but the current
     branch.

Let's make this more like the fetch-remote config. We'll
record the pushremote for each branch, and then explicitly
compute the correct remote for the current branch at the
time of reading.

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