.gitignore filtering in depot_tools PRESUBMIT.py could be made more universal and reusable. Right now it covers pylint only.

https://chromium.googlesource.com/chromium/tools/depot_tools.git/+blame/1bc7bfe5d9d8360a611d3f73299249e2124c11d7/PRESUBMIT.py#23

The concern is that regexps from .gitignore start with leading / and are matched against relative paths, so this black lists probably do not work. 

Also, on Windows paths need to be translated to forward slashes before matching. This, together with relative path requirement can be refactored into separate Filter component.

It is also hard to test what the internal function that reads .gitignore gets on the output. Connected is the fact that this piece is not covered by tests.