If a contributor email is already in the list we won't create a new
entry. However, if the name is different than in the list, we print a
message if an alias is missing in .mailmap
If an email is not in the list, but the name is in the list this could
mean two things:
1. Two different persons have the same name.
This is possible, but less likely than 2.
2. A contributor used another mail.
This is likely the case and we don't make a new entry,
but print a message.
Use git shortlog to create a list of contributors
that are not yet included in CONTRIBUTORS.rst
The email address is taken as definitive.
People with the same name, but different email addresses
are supported.
Aliases, different email addresses for the same person,
are handled by git with .mailmap
