Though I like the in-built browser based comparison tool of github to compare pull request differences, i would like to use winmerge or any other external tool to compare differences between a pull request of another fork and master. how can i do it without adding the fork as a remote in my local repo
git operations are all locally, so you can't do the diff without adding it as a remote.
Just add it as a remote, fetch it, and then use git difftool to get the diff. If you don't need the remote after that, just git remote rm it and you're done!