[RFC PATCH 0/2] push: add support for pushing to remote groups

This RFC series adds support for `git push` to accept a remote group name (as configured via `remotes.<name>` in config) in addition to a single remote name, mirroring the behaviour that `git fetch` has supported for some time.

A user with multiple remotes configured as a group can now do:

    git push all-remotes

instead of pushing to each remote individually, in the same way that:

    git fetch all-remotes

already works.

The series is split into two patches:

  - Patch 1 moves `get_remote_group`, `add_remote_or_group`, and the
    `remote_group_data` struct out of builtin/fetch.c and into
    remote.c/remote.h, making them part of the public remote API.

  - Patch 2 extends builtin/push.c to use the newly public
    `add_remote_or_group()` to resolve the repository argument as
    either a single remote or a group, and pushes to each member of
    the group in turn.

RFC notes and open questions:

  - The current implementation pushes to group members sequentially.
    A follow-up could add push.parallel (mirroring fetch.parallel) to
    allow parallel pushes to group members via run_processes_parallel(),
    the same mechanism fetch uses. Feedback on whether this is
    desirable for push is welcome.

  - push.default = simple interacts poorly with group pushes when the
    current branch has no upstream set, since setup_default_push_refspecs()
    will die on the first remote that is not the upstream. Users should
    use push.default = current or explicit refspecs for group pushes.
    It is worth discussing whether the group push path should automatically
    imply push.default = current, or whether a clear error message
    directing the user to configure this would be sufficient.

  - force-with-lease semantics across a group push are currently
    unmodified — the same CAS constraints are forwarded to every remote
    in the group. Whether this is the right behaviour or whether
    per-remote lease tracking is needed is an open question.

  - I will also add the tests and documentations in the next iterations

Usman Akinyemi (2):
  remote: move remote group resolution to remote.c
  push: support pushing to a remote group

 builtin/fetch.c | 42 -----------------------
 builtin/push.c  | 89 ++++++++++++++++++++++++++++++++++++++-----------
 remote.c        | 37 ++++++++++++++++++++
 remote.h        | 12 +++++++
 4 files changed, 119 insertions(+), 61 deletions(-)