[ci] skip_unless_changed uses git log, not diff
[ci] skip_unless_changed uses git log, not diff

On PR builds, `pipeline.git.base_revision` is `origin/master`. This means that
currently, when changes to some project are introduced on master, every open PR
will run the builds for that project.

After this merge, only changes introduced in the branch will be considered when
determining which builds should be run.

exponentjs/exponent,exponent/exponent,exponent/exponent,exponent/exponent,exponentjs/exponent,exponentjs/exponent,exponent/exponent,exponent/exponent,exponent/exponent,exponentjs/exponent,exponent/exponent,exponent/exponent,exponent/exponent,exponentjs/exponent,exponentjs/exponent,exponentjs/exponent,exponentjs/exponent,exponentjs/exponent