fix($location): rewrite relative URI correctly if path==='/' in legacy html5Mode

Currently, legacy browsers get to use a clever scheme for resolving relative URIs in html5Mode,
and resolve the URI relative to $location.path().

Currently, $location.path() can be '/' under certain circumstances, which means that when we
split $location.path() on '/' and later join by '/' after adding another path component,
we end up with '//pathComponent'. $$rewrite fails to deal with this correctly, and effectively
the $location is never changed from the root path.

This CL corrects this by ensuring that the duplicate '/' situation does not occur when resolving
relative URIs.

Closes #8684

src/ng/location.js

@@ -682,6 +682,7 @@ function $LocationProvider(){
             // relative path - join with current path
             var stack = $location.path().split("/"),
               parts = href.split("/");
+            if (stack.length === 2 && !stack[1]) stack.length = 1;
             for (var i=0; i<parts.length; i++) {
               if (parts[i] == ".")
                 continue;

test/ng/locationSpec.js

@@ -954,6 +954,20 @@ describe('$location', function() {
     });
 
 
+    it('should produce relative paths correctly when $location.path() is "/" when history enabled on old browser', function() {
+      configureService('partial1', true, false, true);
+      inject(
+        initBrowser(),
+        initLocation(),
+        function($browser, $location) {
+          $location.path('/');
+          browserTrigger(link, 'click');
+          expectRewriteTo($browser, 'http://host.com/base/index.html#!/partial1');
+        }
+      );
+    });
+
+
     it('should rewrite abs link to hashbang url when history enabled on old browser', function() {
       configureService('/base/link?a#b', true, false);
       inject(