Batches: populate branch name during template resolution

Created by: camdencheek

During batch execution, we were creating the template variable with no branch name set, which meant that steps would just get an empty string for the branch name if it is used in a step.

Fixes https://github.com/sourcegraph/sourcegraph/issues/59250

Test plan

Ran a batch change that uses the ${{ repository.branch }} template variable and ensured that the variable gets populated with a non-empty string.