submit: Check base CR status before submission

[ed-irl]

Adds a pre-submit check that walks the downstack of the branch being
submitted and queries the forge for the state of each base's change.
If a base branch's change has already been merged, submit is blocked
with a message pointing the user at gs repo sync to recover.

The check applies to branch submit, stack submit, upstack submit,
and downstack submit. It is skipped when no supported forge is
configured, or --force is used in cases where the user knows the local state
is intentionally divergent.

Summary

• Add the check in the submit handler for single branch and batch submissions.
• For batch submissions, load the branch graph in advance and re-use it,
  as it's required anyway for the other operations.
• Refactor upstack submit to extract base-submitted check into a
  helper for readability.

Test plan

Unit tests and test scripts verify behavior.

[ed-irl]

This change is part of the following stack:

• submit: Check base CR status before submission #1154 ◀
  • sync: retarget upstack PRs on forge after deletion #1150
    • fix: resolve BranchOnto target from git ref, not stored state #1151
      • Add 'downstack merge' command to merge stacks #1152
        • feat: add 'gs ci merge-guard' to block out-of-order stacked PR merges #1153

_{Change managed by git-spice.}

[abhinav]

Thanks, @ed-irl!
I'll try to give this stack time over this week/weekend.