R1:d1649d176f67

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…