fix(cmd): fix with_stdout implementation

Admittedly this fix is solely based on the documentation provided
for this parameter, which indicated a different intend than was
actually implemented. Also I don't believe doing this will cause
any harm.

As a special note: the call to `open(os.devnull, 'wb')` does not seem leak
the handle, apparently it is given as-is to the subprocess, which will then
close it naturally. This was tested using an interactive session via `htop`
on osx.

Fixes #437

Author: Byron

doc/source/changes.rst

@@ -7,6 +7,8 @@ Changelog
 
 * Fix: progress handler exceptions are not caught anymore, which would usually just hide bugs
   previously.
+* Fix: The `Git.execute` method will now redirect `stdout` to `devnull` if `with_stdout` is false, 
+  which is the intended behaviour based on the parameter's documentation.
 
 2.0.2 - Fixes
 =============

git/cmd.py

@@ -605,7 +605,7 @@ def execute(self, command,
                          bufsize=-1,
                          stdin=istream,
                          stderr=PIPE,
-                         stdout=with_stdout and PIPE or None,
+                         stdout=with_stdout and PIPE or open(os.devnull, 'wb'),
                          shell=self.USE_SHELL,
                          close_fds=(os.name == 'posix'),  # unsupported on windows
                          **subprocess_kwargs