github: optimize style workflow

Only fetch the commits we need instead of fetching the entire history.

Unfortunately there doesn't seem to be a way to add 1 to the number of
commits without an extra step, so do it in a new step and pass the
information onto $GITHUB_ENV so it can be used later.

Signed-off-by: Ahmad Khalifa <ahmadkhalifa570@gmail.com>
Reviewed by: imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/1538