Status() reports Ahead = 0, Behind = 0 when the upstream branch is gone, so unpushed commits look in sync #177

What's wrong

GitStatusParser only sets ahead and behind when it reads a # branch.ab header, and both default to 0 (GitIntegration/Parsing/GitStatusParser.cs:130-136). GitStatus.Ahead and GitStatus.Behind are plain int, and nothing records that the upstream is missing (GitIntegration/Models/GitStatus.cs:22-28).

Porcelain v2 prints # branch.upstream origin/feat but leaves out # branch.ab entirely when the configured upstream ref does not exist. That happens when:

• the remote branch was deleted after a merge, or
• the upstream was configured but never fetched.

Failure scenario (reproduced with git 2.43)

1. Push branch feat with -u, then make one more local commit.
2. Run git push origin --delete feat.
3. git status --porcelain=v2 --branch prints # branch.oid …, # branch.head feat, # branch.upstream origin/feat, and no branch.ab line. (git status -sb shows [gone].)
4. Status() returns Upstream = origin/feat, Ahead = 0, Behind = 0.

A caller that checks Upstream != null && Ahead == 0 concludes there is nothing to push. It may then discard the branch or worktree and lose the unpushed commit. That is exactly the "is it safe to throw this away" question the library is meant to answer.

Suggested fix

Either:

• make Ahead/Behind int?, null when branch.upstream is present without branch.ab; or
• add bool IsUpstreamGone (true when branch.upstream is present without branch.ab), and document that Ahead/Behind are meaningless while it is true.

Acceptance criteria

• A parser test covers headers with branch.upstream and no branch.ab.
• An integration test deletes the remote branch and asserts that the gone state is reported, not 0/0.