Try to find the appropriate SHAs

Try to find the appropriate SHAs

We want all the patches between the branch-off point from
blead and the current branch. The current branch position lives in
$ENV{GITHUB_SHA}, and the (latest) branch-off point from blead is
found by running

    git merge-base $ENV{GITHUB_BASE_REF} $ENV{GITHUB_HEAD_REF}

We don't want to use the last commit, as it's a merge commit.

That commit will also have the Github mail address, not the real
git mail address of the author . Instead we use the (second)
parent commit to that merge commit.