use @@Perl@@ in built scripts

Several of the built shell commands invoke a bare "perl" to
perform some one-liners. This will use the first perl in the
PATH rather than the one specified by the user's SHELL_PATH.
We are not asking these perl invocations to do anything
exotic, so typically any old system perl will do; however,
in some cases the system perl may have unexpected behavior
(e.g., by handling line endings differently). We should err
on the side of using the perl the user pointed us to.

The downside of this is that on systems with a sane perl
setup, we no longer find the perl at runtime, but instead
point to a static perl (like /usr/bin/perl). That means we
will not handle somebody moving perl without rebuilding git,
whereas before we tracked it just fine. This is probably not
a big deal, though, as the built perl scripts already
suffered from this.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-am.sh

@@ -302,7 +302,7 @@ split_patches () {
 			# not starting with Author, From or Date is the
 			# subject, and the body starts with the next nonempty
 			# line not starting with Author, From or Date
-			perl -ne 'BEGIN { $subject = 0 }
+			@@PERL@@ -ne 'BEGIN { $subject = 0 }
 				if ($subject > 1) { print ; }
 				elsif (/^\s+$/) { next ; }
 				elsif (/^Author:/) { s/Author/From/ ; print ;}
@@ -334,7 +334,7 @@ split_patches () {
 			# Since we cannot guarantee that the commit message is in
 			# git-friendly format, we put no Subject: line and just consume
 			# all of the message as the body
-			LANG=C LC_ALL=C perl -M'POSIX qw(strftime)' -ne 'BEGIN { $subject = 0 }
+			LANG=C LC_ALL=C @@PERL@@ -M'POSIX qw(strftime)' -ne 'BEGIN { $subject = 0 }
 				if ($subject) { print ; }
 				elsif (/^\# User /) { s/\# User/From:/ ; print ; }
 				elsif (/^\# Date /) {

git-instaweb.sh

@@ -581,7 +581,7 @@ EOF
 
 gitweb_conf() {
 	cat > "$fqgitdir/gitweb/gitweb_config.perl" <<EOF
-#!/usr/bin/perl
+#!@@PERL@@
 our \$projectroot = "$(dirname "$fqgitdir")";
 our \$git_temp = "$fqgitdir/gitweb/tmp";
 our \$projects_list = \$projectroot;

git-request-pull.sh

@@ -96,7 +96,7 @@ find_matching_ref='
 	}
 '
 
-ref=$(git ls-remote "$url" | perl -e "$find_matching_ref" "$head" "$headrev" "$tag_name")
+ref=$(git ls-remote "$url" | @@PERL@@ -e "$find_matching_ref" "$head" "$headrev" "$tag_name")
 
 url=$(git ls-remote --get-url "$url")
 

git-submodule.sh

@@ -115,7 +115,7 @@ module_list()
 		git ls-files --error-unmatch --stage -- "$@" ||
 		echo "unmatched pathspec exists"
 	) |
-	perl -e '
+	@@PERL@@ -e '
 	my %unmerged = ();
 	my ($null_sha1) = ("0" x 40);
 	my @out = ();