perl/Makefile.PL: warn about duplicate module list in perl/Makefile

Adding or removing a module requires modifying both files to support
builds with and without MakeMaker.  Add a comment to remind patch
authors and reviewers at the crucial moment.

Longer term, it would be nicer to maintain a single list, perhaps in a
separate file used by both build systems.

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

perl/Makefile.PL

@@ -24,6 +24,10 @@ endif
 MAKE_FRAG
 }
 
+# XXX. When editing this list:
+#
+# * Please update perl/Makefile, too.
+# * Don't forget to test with NO_PERL_MAKEMAKER=YesPlease
 my %pm = (
 	'Git.pm' => '$(INST_LIBDIR)/Git.pm',
 	'Git/I18N.pm' => '$(INST_LIBDIR)/Git/I18N.pm',