Count the beta prerelease number just from master
Count the beta prerelease number just from master

We were computing a merge-base between the remote beta and master
branches, but this was giving incorrect answers for the first beta if
the remote hadn't been pushed yet. For instance, `1.45.0-beta.3359`
corresponds to the number of merges since the 1.44 beta, but we really
want just `.1` for the sole 1.45 beta promotion merge.

We don't really need to query the remote beta at all -- `master..HEAD`
suffices if we assume that we're on the intended beta branch already.

graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust