Fix press 'd' (REQ_VIEW_DIFF) on LINE_STAT_UNSTAGED and LINE_STAT_STAGED

pressing 'd' shows actual diff of stage/unstaged line instead of just a blank diff with "[diff] 0000000000000000000000000000000000000000" in the status bar.


i just looked at the code for some minutes and found this code path to work. if there is a better on, which i'm sure, please re-implement it there. :)