Commit 584fa9cc introduced the global variable diff_empty_count, which
is used in diff.tcl. This variable wasn't declared anywhere which
resulted in an ugly error message box instead of the intended
informative message.
Signed-off-by: NJoerg Bornemann <joerg.bornemann@web.de>
Signed-off-by: NShawn O. Pearce <spearce@spearce.org>