Handle Git wildmatch edge cases

[jdpace]

This PR copies over some edge case handling from the Python pathspec library (cpburnz/python-pathspec#8) that was introduced after this gem was ported.

The original PR goes into more detail but here's an overview of the edge cases:

• / does NOT match all paths the repo
• /abs/path matches: abs/path/foo
• spam matches: spam/, spam/foo
• spam/two does NOT match: foo/spam/two
• spam/** does NOT match: foo/spam/bar
• left/**/right does NOT match: foo/left/bar/right
• **/spam does match: foo/spam/, foo/spam/bar
• foo-*-bar does match: foo-hello-bar/baz
• ~temp-* does match: ~temp-foo/bar
• *.py does match: bar.py/baz

[jdpace]

pry-debugger was removed since it doesn't work with Ruby 2.X.

[highb]

Thanks for submitting this PR! Hm, it looks like something funky is happening in Travis with Ruby 1.8. I'll try checking that out when I have a minute.

[highb]

I didn't realize Ruby 1.8 was so deprecated now. I've updated the Travis testing to the current stable/maintenance Rubies, so I really don't care about the Ruby 1.8 failure.

Thanks again for putting up this PR! I'll get a new Gem version cut soon.