gitweb.cgi: git_blame2: Allow back-trekking through commits

This patch adds the capability of back-trekking through
commits from git_blame2() as follows:
blame2->commit->blame2->commit->blame2->...->initial commit.

Signed-off-by: Luben Tuikov <ltuikov@yahoo.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>

gitweb/gitweb.cgi

@@ -2022,7 +2022,13 @@ sub git_commit {
 		print " | " . $cgi->a({-href => "$my_uri?" . esc_param("p=$project;a=commitdiff;h=$hash")}, "commitdiff");
 	}
 	print " | " . $cgi->a({-href => "$my_uri?" . esc_param("p=$project;a=tree;h=$co{'tree'};hb=$hash")}, "tree") . "\n" .
-	      "<br/><br/></div>\n";
+		"<br/>\n";
+	if (defined $file_name && defined $co{'parent'}) {
+		my $parent = $co{'parent'};
+		print $cgi->a({-href => "$my_uri?" . esc_param("p=$project;a=blame;hb=$parent;f=$file_name")}, "blame") . "\n";
+	}
+	print "<br/></div>\n";
+
 	if (defined $co{'parent'}) {
 		print "<div>\n" .
 		      $cgi->a({-href => "$my_uri?" . esc_param("p=$project;a=commitdiff;h=$hash"), -class => "title"}, esc_html($co{'title'}) . $ref) . "\n" .