Check GitHub for new commits after startup

[jerryorr]

If Strider is down when a commit is pushed to GitHub, Strider won't get the web hook post. When Strider starts up, it'd be nice if it checked GitHub for new commits and run the job if it finds any.

[jaredly]

that's a good idea.

[niallo]

Agreed. Here's some of my initial thoughts:

• Do we just run one job for HEAD if there have been commits since last job, or try to queue multiple jobs for each push? (I think HEAD makes sense - KISS)
• Is this a configurable knob? Is it on by default? (I would say yes and yes)
• How to make this work with manual setup private repos? (May not have OAuth2 token to ping the API - might have to have worker do it using git CLI)