The label currently looks identical in the unambiguous case
("Rebase onto base branch (develop)") and the ambiguous case where
develop is just the config-order tiebreak; pressing 'b' would then
surprise the user with a picker. Show "pick: main, develop" in the
parenthetical when the resolver reports the base is ambiguous so the
upcoming prompt is no longer a surprise.
The new PickBaseBranchLabel i18n string lives next to the existing
PickBaseBranchTitle/Prompt so the disambiguation UI is grouped in
one place.