ShortBranchName previously turned "refs/remotes/origin/main" into
"origin/main", leaking the resolved-ref shape into UI labels that
the user thinks of as plain "main" — the name they put in their
mainBranches config. With the ambiguous-base label now potentially
listing several branches ("pick: origin/main, origin/13"), the noise
is even more pronounced. Strip the remote name along with the
"refs/remotes/" prefix so the short name matches what the user
configured.
Rename the helper to BaseBranchDisplayName to make the constraint
explicit at every call site: dropping the remote is a sensible
choice only for base-branch display, not for refs in general. All
current callers happen to be base-branch related, so the rename is
just a scope-tightening.
Existing integration test updated to expect the bare "master" form.
6e74cc0268