bug#8901: C-u C-<space> fails inside isearch.

[Alan Mackenzie]

In the bzr head:

In any buffer, 
1. Initiate an isearch with C-s
2. Type in incremental characters, taking point further into the buffer.
3. Terminate the search with C-u C-<space>

This should terminate the search and return to where the search started.
Instead, it pushes another mark, as though the C-u weren't typed.

This bug was also present in Emacs 23.2.  It was not present in 22.2.

-- 
Alan Mackenzie (Nuremberg, Germany).