fix(branch onto): rebase loop on conflicts

[abhinav]

Fixes an issue reported by a user where the following steps cause
a rebase conflict loop:

• branches A and B share a base, or the base for A is downstack from B
• branch A is moved on top of B, and the operation runs into a conflict
• the user resolves the conflict and runs gs rebase continue
• they are asked to resolve more conflicts, as if replaying all commits

The cause of this issue is this:
when the operation to move A onto B runs into a conflict,
git-spice's internal state is not updated to reflect the new base
(this is correct and desirable).

However, when the operation is re-run after resolving the conflict,
it attempts to replay commits Base..A onto B,
except Base..B overlaps with that range with the new state of the repository.
This forces the user to resolve a bunch of conflicts from replaying those operations,
but the rebase --continue does not ever succeed, and the state is never updated.
so it keeps retrying the same commits on top of B over and over again.

The fix for this is fairly straightforward:
if B is in the Base..A commit range, rebase only B..A instead of Base..A.

[abhinav]

This change is part of the following stack:

• fix(branch onto): rebase loop on conflicts #582 ◀

_{Change managed by git-spice.}