Encode the environment variables passed to git

Windows allows the environment to have unicode values.
This will cause Python to fail to execute the command.

Change-Id: I37d922c3d7ced0d5b4883f0220346ac42defc5e9
Signed-off-by: Shawn O. Pearce <sop@google.com>

subcmds/forall.py

@@ -151,11 +151,11 @@ terminal and are not redirected.
     first = True
 
     for project in self.GetProjects(args):
-      env = dict(os.environ.iteritems())
+      env = os.environ.copy()
       def setenv(name, val):
         if val is None:
           val = ''
-        env[name] = val
+        env[name] = val.encode()
 
       setenv('REPO_PROJECT', project.name)
       setenv('REPO_PATH', project.relpath)