Fix incorrect assumption that git_root_dir is populated and allow GitGotoDiff to work with Git Custom commands
Sanity-checking git_root_dir and if it's not valid for the current hunk's file, prompt the user to fix it rather than just failing.
Stripping whitespace off filename to fix problem with filename with spaces.
Merge pull request #3 from vanrijn/allow-git-goto-diff-to-operate-on-context-instead-of-name

Fix incorrect assumption that git_root_dir is populated and allow GitGot...