Allow us to read, and default to read, mailmap files from the tip
of the history in bare repositories. This will help running tools
like shortlog in server settings.
* jk/mailmap-from-blob:
mailmap: default mailmap.blob in bare repositories
mailmap: fix some documentation loose-ends for mailmap.blob
mailmap: clean up read_mailmap error handling
mailmap: support reading mailmap from blobs
mailmap: refactor mailmap parsing for non-file sources