When highlighting a commit, using the context menu over the staged changes
and then selecting "Diff this -> selected" the diff was empty.  The same
happened when highlighting the staged changes and using "Diff selected ->
this" over a commit.  The reason was a copy/paste error in [diffcmd].
This fixes it.
Signed-off-by: NJens Lehmann <Jens.Lehmann@web.de>
Signed-off-by: NPaul Mackerras <paulus@samba.org>