Create a new mailmap instance from a repository, loading mailmap files based
on the repository's configuration.
Mailmaps are loaded in the following order:
- '.mailmap' in the root of the repository's working directory, if present.
- The blob object identified by the 'mailmap.blob' config entry, if set.
[NOTE: 'mailmap.blob' defaults to 'HEAD:.mailmap' in bare repositories]
- The path in the 'mailmap.file' config entry, if set.
