Refactor status detection for XLOG segment file in xlogarchive.c (Was: *FF WALs under 9.2)

Hi all,

Except if I am missing something, is there any reason why the sequence used
in XLogArchiveCheckDone and XLogArchiveIsBusy to check if a XLOG segment
has been already archived is duplicated? I guess that doing a little bit of
refactoring here would make sense for simplicity, patch is attached.
Regards,
--
Michael