add-log-file-name-function is a variable defined in `add-log.el'.
Its value is

  • This variable may be risky if used as a file-local variable.

If non-nil, function to call to identify the filename for a ChangeLog entry.
This function is called with one argument, the value of variable
`buffer-file-name' in that buffer. If this is nil, the default is to
use the file's name relative to the directory of the change log file.

You can customize this variable.