contrib: Add script to help with customizing a git checkout for use with GCC

This patch is intended to help with folks setting up a git work
environment for use with GCC following the transition to git.  It
currently does a couple of things.

1) Add an alias 'svn-rev' to git so that you can look up a legacy
commit by its svn revision number.  This enables you to type
	git svn-rev 1234
and git will show the commit log entry relating to SVN r1234.

2) Sets up tracking information for the user's personal area in
the git repo.  It tries to figure out some sensible answers to the
data it needs, but allows the user to override the values.  It then
creates the fetch and push entries that are needed for tracking the
extra refs. This implements one part of the recommendations that I've
proposed in svnwrite.html for dealing with private branches.

It should be possible to run the script more than once and for it to
DTRT.  If you change your answers the configuration should be
correctly updated.

2020-01-13  Richard Earnshaw  <rearnsha@arm.com>

    * gcc-git-customization: New file.