gitk: Fix context menu items for generating diffs when in tree mode

gitk: Fix context menu items for generating diffs when in tree mode

Currently, if you invoke the "diff this -> selected" or "diff selected
-> this" and gitk is in "Tree" mode rather than "Patch" mode, the
diff display pane will just show the header but not the actual diff,
unless gitk has done the diff before and thus has the list of files
that differ.  This was because the logic in gettreediffline that
checked whether we had moved on to doing something else checked the
mode (Tree or Patch) before checking whether the ids we're diffing
had changed.

This fixes it.  The new logic in gettreediffline is slightly hacky
and relies on the fact that the Tree/Patch mode only applies when
we're looking at a single commit, not at the diff between two commits.

Signed-off-by: Paul Mackerras <paulus@samba.org>