Generate much of the CREDITS file from git log

* CREDITS.in: New file, derived from CREDITS, without names of
those who are listed as git log 'Author:'s.
* CREDITS: Remove file.
* Makefile.am [MAINTAINER_MODE] (CREDITS): New rule.
* .gitignore: Add CREDITS.
* .mailmap: New file, required to map git author names and email
addresses to canonical/preferred form.
5 files changed