Fix Windows compatibility for gitignore option

[barak007]

Fixes #153

[sindresorhus]

This needs a test.

[barak007]

Sorry for the description-less PR. I was in a harry and forgot to write a description.
This should fix a case that ensureAbsolutePathForCwd received a cwd Windows path and a path to check which is also Windows path.
It was clear that if you only apply slash to one of them it wont work.

The whole slashes approach is usually not that promising. but I couldn't change the entire codebase.