first_commit_date wrong in template, and should be overridable

I'm migrating old posts to Gekyll, so I'd like to be able to set the value of first_commit_date manually.

Alternatively, one could just add an optional "original_date" to the front matter for old posts, but I personally prefer to keep as much logic out of the template as possible. 

Drawback is that the oldest item in the revision history will still represent the date of the migration, but this can be noted in the initial commit message or merely tolerated.

Also, ```sample-layouts/repo.html``` had an old value, "start_date", for this variable.