Write the base commit to .git/base_sha
Write the base commit to .git/base_sha

This is useful for when we need to know the SHA-1 of the base branch when the build ran which isn't necessarily the HEAD of `master` (or whatever the base branch is).
jtarchie/pullrequest-resource,jtarchie/pullrequest-resource,jtarchie/github-pullrequest-resource,jtarchie/github-pullrequest-resource