magit-file-log: use magit-refresh-log-buffer instead of magit-refresh-file-log-buffer

In `magit-file-log` use `magit-refresh-log-buffer` instead of the
"specialized" `magit-refresh-file-log-buffer`.

The latter refresher was wrong for several reasons:

* There ain't no "file log" buffer - file log uses the same buffer
  as "regular log".  Whether that is a good idea is a different
  issue.

* The file-log refresher effectively re-implemented the regular
  refresher but in a less feature full and buggy fashion.  The person
  who implemented the file-log refresher should instead just have made
  the, rather minimal, required adjustments to the log refresher and
  then have used that.