Add check on version bump

In mkit's world, last change before release (ie. the commit that gets
tagged) must be nothing but version bump in "config.mk"

utils/mkit/include/release.sh

     local newtag=v$newver
     local higher=$(echo -e "$oldtag\n$newtag" | sort -V | tail -n1)
 
+    test "$lastfile" = config.mk \
+     || die "last change must be version bump in config.mk"
+
     test -n "$lasttag" \
      || die "cannot find last tag"