When generating manpages, delete outdated targets first.

This makes "make doc" work even if you made "sudo make doc" previously
by mistake.  Apparently an oversight: the other targets did this already.

Signed-off-by: David Kastrup <dak@gnu.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/Makefile

@@ -120,6 +120,7 @@ clean:
 	mv $@+ $@
 
 %.1 %.5 %.7 : %.xml
+	$(RM) $@
 	xmlto -m callouts.xsl man $<
 
 %.xml : %.txt