Merge branch 'ua/push-remote-group' into seen

"git push" learned to take a "remote group" name to push to, which
causes pushes to multiple places, just like "git fetch" would do.

* ua/push-remote-group:
  push: support pushing to a remote group
  remote: move remote group resolution to remote.c
  remote: fix sign-compare warnings in push_cas_option

diff --cc builtin/fetch.c
Simple merge

diff --cc remote.c
Simple merge

diff --cc remote.h
Simple merge

diff --cc t/meson.build
Simple merge