Project-level ignore

As in this comment

> It would be awesome to have an option "Add To Ignore List" which allows to choose "add to .gitignore" or "add to .git/info/exclude (will be ignored at this computer only)".

When we right-click on a file to ignore it, it should give the option of ignoring it in a .gitignore (that gets committed along with the repo) or .git/info/exclude (which is unique to our computer) or a global gitignore for all repos (garbage files like Thumbs.db. etc)