convert: config option to control Git committer actions When converting a Git repository to Mercurial at Mozilla, I encountered a scenario where I didn't want `hg convert` to automatically add the "committer: <committer>" line to commit messages. While I can hack around this by rewriting the Git commit before it is fed into `hg convert`, I figured it would be a useful knob to control. This patch introduces a config option that allows lots of control over the committer value. I initially implemented this as a single boolean flag to control whether to save the committer message. But then there was feedback that it would be useful to save the committer in extra data. While this patch doesn't implement support for saving in extra data, it does add a mechanism for extending which actions to take on the committer field. We should be able to easily add actions to save in extra data. Some of the implemented features weren't asked for. But I figured they could be useful. If nothing else they demonstrate the extensibility of this mechanism.