Fix Git.execute shell use and reporting bugs

This updates the shell variable itself, only when it is None, from
self.USE_SHELL.

(That attribute is usually Git.USE_SHELL rather than an instance
attribute. But although people probably shouldn't set it on
instances, people will expect that this is permitted.)

Now:

- USE_SHELL is used as a fallback only. When shell=False is passed,
  USE_SHELL is no longer consuled. Thus shell=False always keeps a
  shell from being used, even in the non-default case where the
  USE_SHELL attribue has been set to True.

- The debug message printed to the log shows the actual value that
  is being passed to Popen, because the updated shell variable is
  used both to produce that message and in the Popen call.

Author: EliahKagan

git/cmd.py

@@ -974,6 +974,8 @@ def execute(
         istream_ok = "None"
         if istream:
             istream_ok = "<valid stream>"
+        if shell is None:
+            shell = self.USE_SHELL
         log.debug(
             "Popen(%s, cwd=%s, universal_newlines=%s, shell=%s, istream=%s)",
             redacted_command,
@@ -992,7 +994,7 @@ def execute(
                     stdin=istream or DEVNULL,
                     stderr=PIPE,
                     stdout=stdout_sink,
-                    shell=shell is not None and shell or self.USE_SHELL,
+                    shell=shell,
                     close_fds=is_posix,  # unsupported on windows
                     universal_newlines=universal_newlines,
                     creationflags=PROC_CREATIONFLAGS,