entire-thread: determine current position before resorting.

entire-thread: determine current position before resorting.

Previously oldcur was determined after nm_read_entire_thread(),
leading to entire-thread unintentionally jumping to the first message
in the thread.

Before 918885f32db41 that used to work, because mutt_sort_headers()
wasn't called from within nm_read_entire_thread().

Fix by moving determination of pre nm_read_entire_thread() count a few
lines up. Also reuse existing oldcount variable, rather than
'oc' (which could also be oldcur).

Fixes: #1714