.gitattributes: use the "perl" differ for Perl

As noted in gitattributes(5) this gives better patch context for these
types of files.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/.gitattributes b/.gitattributes
index 482af05..aa08dd2 100644
--- a/.gitattributes
+++ b/.gitattributes
@@ -1,9 +1,9 @@
 * whitespace=!indent,trail,space
 *.[ch] whitespace=indent,trail,space diff=cpp
 *.sh whitespace=indent,trail,space eol=lf
-*.perl eol=lf
-*.pl eof=lf
-*.pm eol=lf
+*.perl eol=lf diff=perl
+*.pl eof=lf diff=perl
+*.pm eol=lf diff=perl
 /Documentation/git-*.txt eol=lf
 /command-list.txt eol=lf
 /GIT-VERSION-GEN eol=lf