Detect when `git subtree` not required

[patcon]

Currently, we run through every project and do a subtree split, which is incredibly time-consuming. Would be great to take this approach:

1. Get stale drupal/ submodule commit
2. Pull most recent changes to 8.x branch
3. Get current drupal/ submodule commit
4. Before doing a git subtree split, check whether any changes occurred between those commits in the module/theme in question.

ie. git diff 9e49307 5a8a967 core/modules/options