Make templates honour SHELL_PATH and PERL_PATH

The hook script templates were hard coded to use /bin/sh and perl.
This patch ensures that they use the same tools specified for the rest
of the suite.

The impetus for the change was noticing that, as shipped, some of the
hooks used shell constructs that wouldn't work under Solaris' /bin/sh
(eg: $(cmd...) substitutions).

Signed-off-by: Ben Walton <bwalton@artsci.utoronto.ca>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

templates/hooks--pre-rebase.sample

@@ -65,7 +65,7 @@ then
 	fi
 else
 	not_in_next=`git-rev-list --pretty=oneline ^${publish} "$topic"`
-	perl -e '
+	@PERL_PATH@ -e '
 		my $topic = $ARGV[0];
 		my $msg = "* $topic has commits already merged to public branch:\n";
 		my (%not_in_next) = map {