i18n: "make distclean" should clean up after "make pot"

This is in "make distclean" and not "make clean" to avoid needlessly
changing the POT-Creation-Date in the following scenario:

	make clean;	# cleaning up after an old build
	git pull
	make pot;	# regenerate po template if necessary
	msgmerge po/my_language.po po/git.pot

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Makefile

@@ -2340,6 +2340,7 @@ dist-doc:
 
 distclean: clean
 	$(RM) configure
+	$(RM) po/git.pot
 
 clean:
 	$(RM) *.o block-sha1/*.o ppc/*.o compat/*.o compat/*/*.o xdiff/*.o vcs-svn/*.o \