push: support pushing to a remote group

push: support pushing to a remote group

`git fetch` accepts a remote group name (configured via `remotes.<name>`
in config) and fetches from each member remote. `git push` has no
equivalent — it only accepts a single remote name.

Teach `git push` to resolve its repository argument through
`add_remote_or_group()`, which was made public in the previous patch,
so that a user can push to all remotes in a group with:

    git push <group>

When the argument resolves to a single remote, the behaviour is
identical to before. When it resolves to a group, each member remote
is pushed in sequence.

The group push path rebuilds the refspec list (`rs`) from scratch for
each member remote so that per-remote push mappings configured via
`remote.<name>.push` are resolved correctly against each specific
remote. Without this, refspec entries would accumulate across iterations
and each subsequent remote would receive a growing list of duplicated
entries.

Mirror detection (`remote->mirror`) is also evaluated per remote using
a copy of the flags, so that a mirror remote in the group cannot set
TRANSPORT_PUSH_FORCE on subsequent non-mirror remotes in the same group.

Suggested-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Usman Akinyemi <usmanakinyemi202@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>