Respect .mailmap

[xxfogs]

Is your feature request related to a problem? Please describe.
Historical botched or changed names and emails cannot be easily (and with little consequences) changed in git other than by the use of .mailmap, which gitui has not yet implemented.

Describe the solution you'd like
.mailmap bindings to be respected in the git history.

Describe alternatives you've considered
None. .mailmap is a part of the git CLI.

Additional context

• https://git-scm.com/docs/gitmailmap

[extrawurst]

all needed bindings are in git2, this should be fairly streightforward to be implemented.