gitweb: Improve "next" link in commitdiff view

Check if 'hp' (hash_parent) parameter to 'commitdiff' view is one of
'h' (hash) commit parents, i.e. if commitdiff is of the form
"<commit>^<n> <commit>", and mark it as such in the bottom part of
navigation bar. The "next" link in commitdiff view was introduced
in commit 151602df00

If 'hb' is n-th parent of 'h', show the following at the bottom
of navigation bar:
  (from parent n: _commit_)

Signed-off-by: Jakub Narebski <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

gitweb/gitweb.perl

@@ -4479,7 +4479,14 @@ sub git_commitdiff {
 				$hash_parent_short = substr($hash_parent, 0, 7);
 			}
 			$formats_nav .=
-				' (from: ' .
+				' (from';
+			for (my $i = 0; $i < @{$co{'parents'}}; $i++) {
+				if ($co{'parents'}[$i] eq $hash_parent) {
+					$formats_nav .= ' parent ' . ($i+1);
+					last;
+				}
+			}
+			$formats_nav .= ': ' .
 				$cgi->a({-href => href(action=>"commitdiff",
 				                       hash=>$hash_parent)},
 				        esc_html($hash_parent_short)) .