`generate-pipeline` uses `master` branch instead of `main`

As you probably know, most of Git repo providers have migrated to `main` branch instead of `master`. Chalice although is using the legacy `master` and it cannot be configured via the command line arguments.

I think we either need to change its main branch to `main` or accept command line args for arbitrarily set the branch which chalice will use to generate the pipeline for.

Start by locating the `generate-pipeline` entry point and the code or tests that define its use of the `master` branch. Confirm whether the intended outcome is a default of `main`, configurable branch arguments, or both; done when the chosen behavior is covered by the relevant pipeline-generation tests.

Written by the indexing model from the issue text.