Cannot create release candidates if there are no engine cherrypicks #172179
Description
As part of work on release automation, we added a script called last_engine_commit.sh:
flutter/bin/internal/last_engine_commit.sh
Lines 6 to 29 in edf4758
The tl;dr is that we have a few validation steps (one in recipes, one in presubmit) that makes sure the contents of bin/internal/engine.version represent the last commit that "touched" the engine, if any. That "if any" is doing work, because it seems like, up until now, we've always had at least one engine cherrypick (yay Impeller on Android launch!). Now that (for 3.35) we don't have one, we'll be blocked from making a release because, technically, there have been no changes to the engine, and the last engine SHA is pre-branch, which is not valid (it's not a release build).
The script should be updated to reflect:
- If the engine has been changed since the branch cut, use that SHA
- Otherwise use the first SHA appears on the branch (i.e. that one that sets
bin/internal/release-candidate-branch.version)