bug#29215: 26.0; `dired-isearch-filter-filesnames`: be able to match prefix

[Drew Adams]

> 1. Typo in first line of doc string:
> 
>    Test whether the current search hit is a file name.
> 
>    It does not test whether the search hit is a file name.
>    It tests whether the search hit is IN a file name.

Actually, even that is incorrect.  The actual behavior is
to test whether ANY PART OF the search hit is in a file
name.  That is, if the search string matches any part of
a file name then this returns non-nil.

For example, if you search for "3  h" and there is a file
name that begins with "h" and its last-modified date, as
displayed in Dired, ends with "3", then this predicate
returns non-nil and the search commands that use the
predicate successfully find that file name.

This is not obvious.