remote.c: introduce a way to have different remotes for fetch/ push

Currently, do_push() in push.c calls remote_get(), which gets the
configured remote for fetching and pushing.  Replace this call with a
call to pushremote_get() instead, a new function that will return the
remote configured specifically for pushing.  This function tries to
work with the string pushremote_name, before falling back to the
codepath of remote_get().  This patch has no visible impact, but
serves to enable future patches to introduce configuration variables
to set this variable.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
remote.c: introduce remote.pushdefault

This new configuration variable defines the default remote to push to,
and overrides `branch.<name>.remote` for all branches.  It is useful
in the typical triangular-workflow setup, where the remote you're
fetching from is different from the remote you're pushing to.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
remote.c: introduce branch.<name>.pushremote

This new configuration variable overrides `remote.pushdefault` and
`branch.<name>.remote` for pushes.  In a typical triangular-workflow
setup, you would want to set `remote.pushdefault` to specify the
remote to push to for all branches, and use this option to override it
for a specific branch.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
WIP: fetch-push: write a test