Find a change log file for M-x add-change-log-entry and return the name.
Optional arg FILE-NAME specifies the file to use.
If FILE-NAME is nil, use the value of `change-log-default-name'.
If `change-log-default-name' is nil, behave as though it were 'ChangeLog'
(or whatever we use on this operating system).
If `change-log-default-name' contains a leading directory component, then
simply find it in the current directory. Otherwise, search in the current
directory and its successive parents for a file so named.
Once a file is found, `change-log-default-name' is set locally in the
current buffer to the complete file name.
Optional arg BUFFER-FILE overrides `buffer-file-name'. (fn &optional FILE-NAME BUFFER-FILE)