t/lib/GitSetup: Enable skipping user setup.

This is necessary for global user configuration failure test.
t/09-no-global-git.t: Test failure of ~/.gitconfig not being set up.

If you have no git.user in ./.git/config and no git.user in ~/.gitconfig,
calling ->_releaser causes git to fatalize in a very unfriendly way
without any context.

Also: Note, I've used Test::Differences here, because Test::Deep was
entirely useless at telling you what was different when it was
different.
Contributors.pm: For _releaser, excute git.config in a try { } block to avoid fataling.

As it is, if there is no user.name anywhere ( either in ~ or ./ ), git
fatalizes.
Contributors.pm: Avoid warning about undef eq $value when $releaser is undef.

Only perform the releaser filter if a releaser is defined.
.mailmap: Add kentnl@cpan.org for future glory.