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.
Merge branch 'PR/1': Test and Fix for `git config user.name` bailing if it doesn't exist.

closes #1
minor formatting tweaks
Dist-Zilla-Plugin-Git-Contributors-0.006

          - increase the minimum required version of Unicode::Collate
          - increase the minimum required version of Git::Wrapper
          - avoid doubly-encoded committer strings on older perls by avoiding
            encoding the string before we send it to IPC::Open3
          - make sure not to crash doing "dzil build" when a user has no
            user.name and include_releaser = 0 (Kent Fredric, #1)