files(ShouldExclude): compatibility with gitignore

[lidel]

Bug extracted from #516, my rough understanding is:

• The current implementation ShouldExclude is not fully compatible with https://git-scm.com/docs/gitignore.

  • [There is an] issue where bar/ didn't catch directories. [Both] bar and bar/ rules [should] filter out the bar directory and its contents.

  • To be fully compatible, the paths have to be matched against a relative path, right now it's just a file name.