doc/RELEASING: Fix the check for diffs from origin/master

The recommended command to check for differences from master to the
remote master was using git log with a range from the local master to
the remote master but this wouldn't work if the local master is ahead
of the remote master because the range is backwards. This patch
changes it to recommend git diff --stat instead because then the
command would work even if the two branches have diverged.

doc/RELEASING

@@ -10,7 +10,7 @@ When making a new release;
 
    and this:
 
-     $ git log master..origin/master
+     $ git diff --stat master origin/master
 
    should be empty. Commit and push before the next step.