bug#25302: 25.1; `isearch-allow-prefix' does not allow `C-u C-u'

[Drew Adams <drew.adams@oracle.com>]

1. emacs -Q

2. Ensure that `isearch-allow-prefix' is non-nil, which it is by default.

3. Search for anything, such as character `e', then hit `C-u C-u':

 C-s e C-u C-u

The second `C-u' terminates Isearch.  It should not.  The raw prefix arg
`(16)' should be passed through to whatever command is invoked by the
next key sequence.

The second `C-u' acts as if `isearch-allow-prefix' were nil.

Note that in Emacs 23, if `isearch-allow-scroll' is non-nil then `C-u
C-u' is passed through OK.  (In Emacs 23, `isearch-allow-prefix' does
not exist.)

But in Emacs 24.4 and later even the workaround of setting
`isearch-allow-scroll' to `t' does not work, to correct the
prefix-arg problem.  The second `C-u' always exits Isearch.

So this is a regression wrt that workaround.  The regression was introduced in Emacs 24.4, which is also when `isearch-allow-prefix'
was introduced.



In GNU Emacs 25.1.1 (x86_64-w64-mingw32)
 of 2016-09-17 built on LAPHROAIG
Windowing system distributor 'Microsoft Corp.', version 6.1.7601
Configured using:
 'configure --without-dbus --without-compress-install CFLAGS=-static'