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>