remote.c: introduce branch.<name>.pushremote

remote.c: introduce branch.<name>.pushremote

This new configuration variable overrides `remote.pushdefault` and
`branch.<name>.remote` for pushes.  When you pull from one
place (e.g. your upstream) and push to another place (e.g. your own
publishing repository), 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 <redacted>
Reviewed-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>