Commands for updating timestamps of source files based on `git commit`
times need to run in the correct directory for the git repository in
question; this isn't always the same as the initially cloned repository.
Partial updates require that source files have appropriate timestamps,
which Git does not set in any special way by default. Thus, before
attempting to update files in-place, we set source timestamps based on
last commit time, which is the effective update time in any case.
