Build stats from Drupal origin/HEAD

[fuzzbomb]

The current Drupal core development branch is 8.5.x but Drupalcores is still building the report from the 8.4.x branch.

Other PRs (#89, #96, #105) have addressed this by incrementing the branch number. The problem is that we'll keep having to do this every 6 months, and it can be overlooked.

This PR proposes a different approach, which removes the need to keep updating the branch number every 6 months.

Instead of specifying a branch number during git operations, we can ask for origin/HEAD and assume that ref points to the active development branch. The ref should point to whatever is set as the "default branch" in the Drupal project node settings (i.e. under Edit > Default branch). I assume the core committers update this? The default branch is currently 8.5.x.

This PR does the following:

• Removes the --branch flag from git clone. This will result in a checkout of whichever branch is marked as origin/HEAD in the upstream repo on drupal.org
• Changes the current checkout-and-pull approach to git remote update followed by git checkout origin/HEAD. This results in a detached head state; we're no longer building the stats from a local branch.
• Changes the ref for git log from a branch number to origin/HEAD

[fuzzbomb]

CI test failure looks unrelated to the changes in this PR. The same problem is happening in PRs #104 and #105. Not sure what to do about this, relates to something in the Chalk npm module.

const escapeStringRegexp = require('escape-string-regexp');
^^^^^
SyntaxError: Use of const in strict mode.

[wimleers]

+1 for principle/approach.

[alexpott]

Looks great. +1 too

[lauriii]

Looks good, thanks!