Allow option do disregard global `.mailmap`

[odrotbohm]

I have a global .mailmap set up to make sure Git understands different email addresses I use throughout my projects when performing analyses. That said, to make sure I (have) use(d) the proper email for particular projects, it would be nice to be able to see the unaliased author/ committer name/and email in Fork's commit history. The only way to get this to work currently seems to git config --local mailmap.file /dev/null.

[DanPristupov]

1. Mailmap handling lies at the very core of the program, and making it modifiable would be a huge task.
2. I think this is a very rare case for adding an option for it.