Skip PRs where base.sha == head.sha
Style tweak for comments: newline before comment body
Don't log exceptions: we get enough info from the debug logger
Steal _get_author from the issues migrator

...and do a simple hacky memoisation.
Better support for porting pull requests

The migrator now attempts to label pull requests by ref first (in case the
branches are still around), falling back to labelling by SHA if that doesn't
work.

It will also add a comment to a merged pull request detailing when it was
merged, by whom (if it knows), and in which merge commit.
Restore working directory when migrating git repos
Upgrade to requests v1.1.0