Makefile: consistently use PERL_PATH

When injecting the Perl path into our scripts we sometimes use '@PERL@'
while we othertimes use '@PERL_PATH@'. Refactor the code use the latter
consistently, which makes it easier to reuse the same logic for multiple
scripts.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-request-pull.sh

@@ -112,7 +112,7 @@ find_matching_ref='
 	}
 '
 
-set fnord $(git ls-remote "$url" | @PERL@ -e "$find_matching_ref" "${remote:-HEAD}" "$headrev")
+set fnord $(git ls-remote "$url" | @PERL_PATH@ -e "$find_matching_ref" "${remote:-HEAD}" "$headrev")
 remote_sha1=$2
 ref=$3