t/lib/GitSetup: Enable skipping user setup.

This is necessary for global user configuration failure test.
Contributors.pm: For _releaser, excute git.config in a try { } block to avoid fataling.

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

Additionally, _releaser returning undef results in a warning about
comparing undef using 'eq', and this is avoided by skipping the "remove
releasers" if there are no releasers to remove.

Attached test emulates this scenario with an empty HOME and an
unconfigured ./.git/config.
.mailmap: Add kentnl@cpan.org for future glory.
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