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