Fix goto source for summary diffs

Previously, the goto source shortcut for summary diffs (e.g. diffs showing
all staged changes) would not work. The reason was that
the view data structure member url contained only the path to the project
root. This commit changes

  1. modifies the vcsDiff api diffLineToSource/Target
     to return relative source paths together with source lines
  2. uses the result together with the project root to compute
     the url to the path in the shortcut handler.