Make all Copybara transformations reversible.

Git-on-Borg Code Review: https://ise-crypto-internal-review.git.corp.google.com/c/cloudcryptosdk/+/12320

Expected changes in the Piper->Git export:
 * protobuf_helper.c
 * objc comments that we're previously needlessly transformed

Also, use github_pr_origin as the origin for the github_pr_to_piper workflow with the use_merge flag enabled.

Currently, the github_pr_to_piper workflow results in CLs with a huge amount of spurious diffs. These changes facilitate clean imports of pull requests.

PiperOrigin-RevId: 199864728
GitOrigin-RevId: d4277d33aa70b6a3d1868834cf9e9881b7a83c9b