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 git.user in ~/.gitconfig,
calling ->_releaser causes git to fatalize in a very unfriendly way
without any context.

Attached test emulates this scenario with an empty HOME and an
unconfigured ./.git/config.
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.
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