Merge branch 'jk/mailmap-from-blob'

Merge branch 'jk/mailmap-from-blob'

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