git-multimail: an improved replacement for post-receive-email

Add git-multimail, a tool for generating notification emails for
pushes to a Git repository.  It is largely plug-in compatible with
post-receive-email, and is proposed to eventually replace that script.
The advantages of git-multimail relative to post-receive-email are
described in README.migrate-from-post-receive-email.

git-multimail is organized in a directory contrib/hooks/multimail.
The directory contains:

* git_multimail.py -- a Python module that can generate notification
  emails for pushes to a Git repository.  The file can be used
  directly as a post-receive script (configured via git config
  settings), or it can be imported as a Python module and configured
  via arbitrary Python code.

* README -- user-level documentation for configuring and using
  git-multimail.

* post-receive -- an example of building a post-receive script that
  imports git_multimail.py as a Python module, with an example of how
  to change the email templates.

* README.migrate-from-post-receive-email -- documentation targeted at
  current users of post-receive-email, explaining the differences and
  how to migrate a post-receive-email configuration to git-multimail.

* migrate-mailhook-config -- a script that can migrate a user's
  post-receive-email configuration options to the equivalent
  git-multimail options.

* README.Git -- a short explanation of the relationship between
  git-multimail and the rest of the Git project, plus the exact date
  and revision when this version was taken from the upstream project.

All but the last file are taken verbatim from the upstream
git-multimail project.

git-multimail is originally derived from post-receive-email and also
incorporates suggestions from the mailing list as well as patches by
the people listed below.

Signed-off-by: Michael Haggerty <[email protected]>
Contributions-by: Matthieu Moy <[email protected]>
Contributions-by: Ramkumar Ramachandra <[email protected]>
Contributions-by: Chris Hiestand <[email protected]>
Contributions-by: Michiel Holtkamp <[email protected]>
Contributions-by: Ævar Arnfjörð Bjarmason <[email protected]>
Reviewed-by: Jonathan Nieder <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: mhagger