branch submit matching: account for local changes

[abhinav]

ref: #491 (comment)

When submitting a branch that doesn't have a PR associated with it,
git-spice attempts to detect previously submitted PRs for the same branch
(either by git-spice or manually).
This matching logic expects the PR's HEAD to be the same as the local branch.
If the PR's HEAD is different, git-spice will assume it's a conflicting branch
name and will generate a new upstream branch name.

This breaks a workflow where you:

• check out a PR submitted elsewhere with git-spice
• make a few changes
• try to use git-spice to submit the PR

The PR's head doesn't match the local HEAD because of the local changes.

A possible improvement here is to check the local HEAD's parents. Perhaps:
if PR's HEAD is reachable from the local HEAD,
and there are no other branches in the middle,
then consider it a match.