git-instaweb: fix mod_perl detection for apache2

The script was looking for something that matched the '^our $gitbin'
regex, which no longer exists in gitweb.cgi.

Now it looks for 'MOD_PERL', which should be on the line that checks
to see if the script is running in a mod_perl environment.

Signed-off-by: Mark Rada <marada@uwaterloo.ca>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-instaweb.sh

@@ -278,7 +278,7 @@ EOF
 
 	# check to see if Dennis Stosberg's mod_perl compatibility patch
 	# (<20060621130708.Gcbc6e5c@leonov.stosberg.net>) has been applied
-	if test -f "$module_path/mod_perl.so" && grep '^our $gitbin' \
+	if test -f "$module_path/mod_perl.so" && grep 'MOD_PERL' \
 				"$GIT_DIR/gitweb/gitweb.cgi" >/dev/null
 	then
 		# favor mod_perl if available