start_command: do not clobber cmd->env on Windows code path

Previously, it would not be possible to call start_command twice for the
same struct child_process that has env set.

The fix is achieved by moving the loop that modifies the environment block
into a helper function. This also allows us to make two other helper
functions static.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

compat/mingw.h

@@ -222,9 +222,8 @@ void mingw_open_html(const char *path);
  * helpers
  */
 
-char **copy_environ(void);
+char **make_augmented_environ(const char *const *vars);
 void free_environ(char **env);
-char **env_setenv(char **env, const char *name);
 
 /*
  * A replacement of main() that ensures that argv[0] has a path