Avoid making it look like kill_process works on Windows

This changes the code in Git.execute's local kill_process function,
which it uses as the timed callback for kill_after_timeout, to
remove code that is unnecessary because kill_process doesn't
support Windows, and to avoid giving the false impression that its
code could be used unmodified on Windows without serious problems.

- Raise AssertionError explicitly if it is called on Windows. This
  is done with "raise" rather than "assert" so its behavior doesn't
  vary depending on "-O".

- Don't pass process creation flags, because they were 0 except on
  Windows.

- Don't fall back to SIGTERM if Python's signal module doesn't know
  about SIGKILL. This was specifically for Windows which has no
  SIGKILL.

See #1756 for discussion.

git/cmd.py

@@ -1015,11 +1015,9 @@ def execute(
 
         def kill_process(pid: int) -> None:
             """Callback to kill a process."""
-            p = Popen(
-                ["ps", "--ppid", str(pid)],
-                stdout=PIPE,
-                creationflags=PROC_CREATIONFLAGS,
-            )
+            if os.name == "nt":
+                raise AssertionError("Bug: This callback would be ineffective and unsafe on Windows, stopping.")
+            p = Popen(["ps", "--ppid", str(pid)], stdout=PIPE)
             child_pids = []
             if p.stdout is not None:
                 for line in p.stdout:
@@ -1028,18 +1026,16 @@ def kill_process(pid: int) -> None:
                         if local_pid.isdigit():
                             child_pids.append(int(local_pid))
             try:
-                # Windows does not have SIGKILL, so use SIGTERM instead.
-                sig = getattr(signal, "SIGKILL", signal.SIGTERM)
-                os.kill(pid, sig)
+                os.kill(pid, signal.SIGKILL)
                 for child_pid in child_pids:
                     try:
-                        os.kill(child_pid, sig)
+                        os.kill(child_pid, signal.SIGKILL)
                     except OSError:
                         pass
                 kill_check.set()  # Tell the main routine that the process was killed.
             except OSError:
-                # It is possible that the process gets completed in the duration after timeout
-                # happens and before we try to kill the process.
+                # It is possible that the process gets completed in the duration after
+                # timeout happens and before we try to kill the process.
                 pass
             return