Generate the list of contributors from git

Instead of requiring contributors to update rcm(7), pull the list of
contributors from the git log instead.

The `rcm.7` file has been moved to `rcm.7.mustache`. The `autogen.sh`
command will write `rcm.7` using autocontrib, a quick script included in
this commit.

This commit adds a `.mailmap` file, used by git-shortlog(1) to produce
correct and useful names and email addresses. An initial `.mailmap` has
been added to correct some existing email addresses and duplications.
`.mailmap` is documented in [Documentation/mailmap.txt][mailmap] for
git.

This adds a maintainer dependency on Ruby and the mustache gem. The
autocontrib program is spiked out in Ruby due to my familarity with the
language, but the choice of templating language (mustache) allows
flexibility for switching languages later. This is a dependency
requirement only for people who run `autogen.sh` -- maintainers.

This adds a maintainer dependency on git. Again, this is only for
maintainers, and many (all?) of them need git to get rcm's source
anyway.

Thanks to Patrick Brisbin and George Brocklehurst for the review.

[mailmap]: https://github.com/git/git/blob/6a907786af835ac15962be53f1492f23e044f479/Documentation/mailmap.txt