config.c: handle lock file in error case in git_config_rename_...

config.c: handle lock file in error case in git_config_rename_...

We could rely on atexit() to clean up everything, but let's be
explicit when we can. And it's good anyway because the function is
called the second time in the same process, we're in trouble.

This function should not affect the successful case because after
commit_lock_file() is called, rollback_lock_file() becomes no-op,
as long as it is initialized.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>