Make XLogFileCopy() look the same as in 9.4.

XLogFileCopy() was changed heavily in commit de768844. However it was
partially reverted in commit 7abc6859 and most of those changes to
XLogFileCopy() were no longer needed. Then commit 7cbee7c0 removed
those unnecessary code, but XLogFileCopy() looked different in master
and 9.4 though the contents are almost the same.

This patch makes XLogFileCopy() look the same in master and back-branches,
which makes back-patching easier, per discussion on pgsql-hackers.
Back-patch to 9.5.

Discussion: 55760844.7090703@iki.fi

Michael Paquier