hg: establish function for performing post-share actions As part of writing an extension that wished to share an arbitrary piece of data among shared repos, I had to reimplement a significant part of hg.share in order to obtain localrepository instances for the source and destination. This patch establishes a function in hg.py that will be called after a share is performed. It is passed localrepository instances so extensions can easily perform additional actions at share time. We move hgrc and shared file writing there because this function is a logical place for it. A side effect of the refactor is writing of the shared file now occurs before updating. This seems more appropriate and shouldn't have any impact on real world behavior.