Test for autocommand that redefines the argument list, when doing ":all".

STARTTEST
:so small.vim
:au BufReadPost Xxx2 next Xxx2 Xxx1
/^start of
A1:.,/end of/w! Xxx1    " write test file Xxx1
$r2:.,/end of/w! Xxx2     " write test file Xxx2
$r3:.,/end of/w! Xxx3     " write test file Xxx3
:next! Xxx1 Xxx2 Xxx3     " redefine arglist; go to Xxx1
:all                      " open window for all args
:w! test.out              " Write contents of Xxx1
:w >>test.out     " Append contents of last window (Xxx1)
:rew                      " should now be in Xxx2
:w >>test.out             " Append contents of Xxx2
:qa!
ENDTEST

start of test file Xxx
	this is a test
	this is a test
	this is a test
	this is a test
end of test file Xxx