.mailmap: tolerate different names, emails in shortlog

Write a conservative .mailmap as an example for contributors to write
their preferred names/ email addresses to.  This patch makes no
assumptions about the same author using different email addresses, and
only collects cases where the full-name of the person is spelt
differently for the same email address.  Verify with:

  $ git shortlog -se | cut -f 2 | cut -d\< -f 2 | sort | uniq -d

It only assumes that the author prefers the full-name that appears
dominantly, by number of contributions.

For the purposes of merging, a strictly alphabetical ordering is
advised.  See MAPPING AUTHORS section of git-shortlog(1) to understand
the format of this file.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>