hgweb: fix regexp for other styles like monoblue

Some styles have indentation.

mercurial/templates/static/mercurial.js

@@ -394,7 +394,7 b' function ajaxScrollInit(urlFormat,'
                 },
                 function onsuccess(htmlText) {
                     if (mode == 'graph') {
-                        var addHeight = htmlText.match(/^<canvas id="graph".*height="(\d+)"><\/canvas>$/m)[1];
+                        var addHeight = htmlText.match(/^\s*<canvas id="graph".*height="(\d+)"><\/canvas>$/m)[1];
                         addHeight = parseInt(addHeight);
                         graph.canvas.height = addHeight;