[linearize] Ignore input commit ref

api.git.checkout uses the input commit's ref unless a ref is specified.
Following the transition from master to main this is now sometimes
'refs/heads/main' rather than master. This caused the recipe to fail to
fetch the linearized repo for builds triggered by Dart SDK commits.

Bug: b/198416551
Change-Id: I4ac04b261a0f9ed68dcc29a115fa975cb92f27d5
Reviewed-on: https://dart-review.googlesource.com/c/recipes/+/213002
Auto-Submit: Alexander Thomas <athom@google.com>
Reviewed-by: Alexander Aprelev <aam@google.com>
Commit-Queue: Alexander Aprelev <aam@google.com>
2 files changed