Use cwd from context instead of kwargs

It is not possible to use cwd argument, since none of the methods called by this
module define cwd explicitly and low-level modules like api.step do not support
it anymore, resulting in a TypeError exception thrown due to unexpected keyword
argument, e.g. see https://ci.chromium.org/swarming/task/3e74a52905a71110. Given
that any users using cwd argument would be broken anyway, it should be safe to
make this change.

Planned usage: https://crrev.com/c/1122874

R=tandrii@chromium.org, iannucci@chromium.org

Bug: 793687
Change-Id: Iadf6bf91d8d41dbc6751a848bb69d7922a206ade
Reviewed-on: https://chromium-review.googlesource.com/1122873
Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
Commit-Queue: Sergiy Byelozyorov <sergiyb@chromium.org>

recipes/recipe_modules/gclient/api.py

@@ -254,7 +254,7 @@ class GclientApi(recipe_api.RecipeApi):
         name = 'recurse (git config %s)' % var
         self(name, ['recurse', 'git', 'config', var, val], **kwargs)
     finally:
-      cwd = kwargs.get('cwd', self.m.path['start_dir'])
+      cwd = self.m.context.cwd or self.m.path['start_dir']
       if 'checkout' not in self.m.path:
         self.m.path['checkout'] = cwd.join(
           *cfg.solutions[0].name.split(self.m.path.sep))