Use git clone instead of git pull to support non-master bases

[itsdalmo]

Ref #35 - Using git clone and specifying the base branch should ensure that the target branch for the PR is available locally when we run rev-parse. In effect, I don't think the previous implementation has ever supported non-master bases.

I've added an E2E test which reproduces the reported error, and does not complain when run against the code in this PR. So hopefully it is fixed 🤞

[itsdalmo]

Ah, now I recall why I used pull instead of clone:
https://blog.github.com/2012-09-21-easier-builds-and-deployments-using-git-over-https-and-oauth/

To avoid writing tokens to disk, don’t clone. Instead, just use the full git URL in your push/pull operations

Need to explore alternate routes.

[itsdalmo]

Switched back to pull since a clone would add the URL for origin in verbatim to the .git/config, which involved writing the access token to disk - no good 😂 The current method should be safe, and it passes all the tests (including E2E).

[colincoleman]

LGTM

[mikael-lindstrom]

LGTM!