The attached patch makes `M-e' (`isearch-edit-string') put point at the first
mismatch position (or the search-string end if no mismatch).

When you hit `M-e' you are ready to edit at the mismatch position.  This makes
it easy to change or insert a char or two to make the string match.  (`C-g', on
the other hand just removes all of the mismatch.)

(This feature complements the use of highlighting to show users where the
mismatch starts.)