Allow modules to alter what commits are fetched

I'm currently integrating open atrium with versioncontrol API. So far its working well except that all of our Drupal sites use Drupal core as a git upstream which means that when ever we import a project repo, we (A) reimport logs that are already imported under another repo and (B) it takes a very log time for each project to be fully imported because of the number or commits to parse into the database.

Now its not that useful to use to be able to go back to 2002 and look at Dries' commits of Drupal, each project only really needs to start having its logs parse from when the project start - of which we know this date.

My patch allows modules to alter the rev-list command in fetchCommits() so that I can add a --since= option to the command, and then I can parse commits since said date instead which will (a) save storage space, (b) take less time and (c) be overall better for my project.