Restore "pseudonyms" feature

[thehale]

Original Feature Description

Pseudonyms

If certain files are being attributed to an unexpected author (e.g. if a
contributor copied code from another project, the blame would show the
contributor instead of the original author), you can manually override the
blame and licensing information using the config/pseudonyms.txt file. Use
one line per override in the following format:

target-path|actual-author|actual-email|license-SPDX-id

All files with a file path containing target-path as a substring will be
attributed to the named actual-author under the named software license.

A list of SPDX license identifiers can be found here:
https://spdx.org/licenses/

Other Notes

This feature is different from #1. Mailmap files map commit authors to normalized values. This feature overrides the commit author for specified file paths.