Feature request: Git `AuthorDate` configuration in `SQUASH` mode

Currently, when Copybara is used in `SQUASH` mode with a Git origin and destination, it keeps the date of the last original commit as the squash commit's `AuthorDate`, but sets the `CommitDate` to the time of its invocation. As far as I can tell, there is no way to change this, leading to needless exposure of metadata from the original repository.

It would be great if there was a way to let it set the `AuthorDate` to the time of invocation as well, or better yet to provide ways of configuring/manipulating these more generally.

Start by locating the Git origin and destination handling for SQUASH mode, then trace how AuthorDate and CommitDate are assigned during invocation. Determine whether the requested behavior can be a focused configuration or requires a broader metadata design. Done means invocation-time AuthorDate can be selected without exposing the original repository's date, with coverage for the SQUASH path.

Written by the indexing model from the issue text.