Topic/gatherdir exclude filename

My pluginbundle now has a share/CONTRIBUTING file, which eventually results in a CONTRIBUTING file being dropped into the root of the dist being built. My bundle does:

    [Git::GatherDir]
    exclude_filename = CONTRIBUTING

to ignore the version already there, to allow it to be regenerated.  But this rule stopped the gathering of the share/ file itself -- because [Git::GatherDir] wraps `\b` around the provided string, and then performs a regex match. This isn't the same as a literal string comparison, which is what [GatherDir] does.

This patch makes [Git::GatherDir] work like [GatherDir] again. Included are tests that demonstrate the fix.

(was also filed as https://github.com/rjbs/dist-zilla-plugin-git/pull/2)