Mark IMPORTED_CHANGE more consistently

Summary:
See <https://github.com/facebook/phabricator/issues/425>. There are some ways that the change parsers may not reach `finishParse()`, but we now need them to in order to mark the commit imported, advance the progress bar, and eventually kick the repository out of IMPORTING status.

Take all the copy/pasted code in the parsers and move it into the parent. Specifically, this is:

  - Printing a status message about starting a parse;
  - checking for bad commits;
  - queueing the next parse stage; and
  - marking the import step complete.

Test Plan: Used `reparse.php --change` to reparse Git, SVN and Mercurial repos.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D7470