Improve new commit check and fix 'first sync' shallow issue

[aormsby]

Fix based on issue #36

Previously, the action checked for new commits rather naively by comparing the heads of the target and upstream branches and looking for a difference. However, that didn't account for the situation where the target branch has new commits unrelated to syncing.

Now, the action loops through all of the commit hashes since the last sync (actually, going back to the shallow_since time), and does a more thorough check to see if a new sync is needed or not.

• shallow_since was added with default 1 month ago
• An unshallow operation was added to deal with 'first sync' situations
• Commit log output was improved