Reset $ENV{HOME} during tests

Do this to avoid depending on ~/.gitconfig.
Drop configuration backwards compatibility

The routine Git::More::_compatibilize_config munged the configuration
date to accomodate very old ways of configuring Git::Hooks. The last
time we changed compatibility was in version 0.031, in January of 2013.

I say it's time to say good-bye to the past and stop carrying this
baggage.

I had to make changes in several test cases because they were still
using old-fashioned configuration, which is bad in itself, since people
may read tests and think they represent standard code.
Implement Git::Hooks::file_temp

This routine returns a File::Temp object representing a temporary file
into which the contents of the file FILE in revision REV has been
copied. It's useful for hooks that need to read the contents of changed
files in order to check anything in them.