The merge tool provides a way for configurations to determine the branches
for a label lazily. This is supported because it allows labels to respect
the currently selected base branch through the Github UI. e.g. if `target: label`
is applied on a PR and the PR is based on the patch branch, then the change
could only go into the selected target branch, while if it would be based on
`master`, the change would be cherry-picked to `master` too. This allows for
convenient back-porting of changes if they did not apply cleanly to the primary
development branch (`master`).
We want to expand this function so that it is possible to report failures if an
invalid target label is appplied (e.g. `target: major` not allowed in
some situations), or if the Github base branch is not valid for the given target
label (e.g. if `target: lts` is used, but it's not based on a LTS branch).
PR Close#38223
6f0f0d3ea2