Support for check-ignore

[sirnewton01]

Check ignore is a useful tool to figure out if a file will be ignored or not given the current ignore configuration.

[driusan]

(.gitignore and .git/exclude also aren't currently implemented, but check-ignore might be a good way to test the logic before implementing them..)

[driusan]

I added https://github.com/driusan/dgit/wiki/Adding-a-git-subcommand if anyone wants to take a stab at this.

[sirnewton01]

In a previous life I implemented the ignore subsystem for another scm system. I'll see if I can brush the dust off of those skills and use them here.

[driusan]

If you're going to look into this, you might want to see if you can replace the globing in #68 with whatever you come up with. (I only implemented the minimal amount I needed to get the test to pass.)

There should probably a some specialized string type to represent patterns, then some methods on that and/or git.File it would be somewhat easier to implement ignores and share the code between other places that need it (like ls-files --exclude-standard)

[sirnewton01]

I made an initial pass on this over the weekend. The changes are in PR #71

[sirnewton01]

The initial implementation is present and covers most of the functionality of check-ignore and the .gitignore format. What remains is the ability to support negation of patterns, which can be covered in another issue.