git-gui: Do not offer to stage three-way diff hunks into the index
git-apply does not accept a patch that was generated as a three-way combined diff format such as we see during merge conflicts. If we get such a diff in our diff viewer and try to send it to git-apply it just errors out and the user is left confused wondering why they cannot stage that hunk.
Instead of feeding a known to be unacceptable hunk to git-apply we now just disable the stage/unstage context menu option if the hunk came from a three way diff. The user may still be confused about why they cannot work with a combined diff, but at least they are only confused as to why git-gui is not offering them the action.