Enhancement

Hey there, first of all thank you all for your work in this project.

Skimming your code in filter_ignore_files() I wondered if instead of grepping you could use git to list files that would be ignored using .git-ftp-ignore .

Consider the following command, this prints out all the files that git would ignore following the rules in .git-ftp-ignore

git ls-files --ignored --cached --exclude-from='.git-ftp-ignore'

I'm not a bash expert, nor git to be honest, but wouldn't it be more convenient to use git to do the ignore path logics and just filter out duplicates with the full list of files with something like uniq?

Just throwing ideas around, I'm really not an expert in bashisms and the like.
I got a bit baffled that my .git-ftp-ignore was ineffective until I RTM and found glob paths are used. Also not able to revert negated paths is quite inconvenient!

Thank you for your time, if the above it's not clear or if I can help with something let me know!

Updated with --cached to comply with git/git@b338e9f.