gitweb: correctly store previous rev in javascript-actions mode

Without this change, the setting

 $feature{'javascript-actions'}{'default'} = [1];

in gitweb.conf breaks gitweb's blame page: clicking on line numbers
displayed in the second column on the page has no effect.

For comparison, with javascript-actions disabled, clicking on line
numbers loads the previous version of the line.

Addresses https://bugs.debian.org/741883.
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Robert Luberda <robert@debian.org>
Acked-by: Jakub Narębski <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

gitweb/static/js/blame_incremental.js

@@ -484,7 +484,7 @@ function processBlameLines(lines) {
 			case 'previous':
 				curCommit.nprevious++;
 				// store only first 'previous' header
-				if (!'previous' in curCommit) {
+				if (!('previous' in curCommit)) {
 					var parts = data.split(' ', 2);
 					curCommit.previous    = parts[0];
 					curCommit.file_parent = unquote(parts[1]);