fix encoding issue

Author: AJMansfield

Diff for: git/cmd.py

@@ -593,6 +593,7 @@ def execute(self, command,
                   command, cwd, universal_newlines, shell)
         try:
             if type(istream) in {str, bytes}:
+                if type(istream) is str: istream = istream.encode('utf-8')
                 proc = Popen(command,
                              env=env,
                              cwd=cwd,