Avoid having a local function seem to be a method

The kill_process local function defined in the Git.execute method
is a local function and not a method, but it was easy to misread as
a method for two reasons:

- Its docstring described it as a method.

- It was named with a leading underscore, as though it were a
  nonpublic method. But this name is a local variable, and local
  variables are always nonpublic (except when they are function
  parameters, in which case they are in a sense public). A leading
  underscore in a local variable name usually means the variable is
  unused in the function.

This fixes the docstring and drops the leading underscore from the
name. If this is ever extracted from the Git.execute method and
placed at class or module scope, then the name can be changed back.

git/cmd.py

@@ -1012,8 +1012,8 @@ def execute(
         if as_process:
             return self.AutoInterrupt(proc, command)
 
-        def _kill_process(pid: int) -> None:
-            """Callback method to kill a process."""
+        def kill_process(pid: int) -> None:
+            """Callback to kill a process."""
             p = Popen(
                 ["ps", "--ppid", str(pid)],
                 stdout=PIPE,
@@ -1046,7 +1046,7 @@ def _kill_process(pid: int) -> None:
 
         if kill_after_timeout is not None:
             kill_check = threading.Event()
-            watchdog = threading.Timer(kill_after_timeout, _kill_process, args=(proc.pid,))
+            watchdog = threading.Timer(kill_after_timeout, kill_process, args=(proc.pid,))
 
         # Wait for the process to return
         status = 0