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.

2. Please provide a fix or enhancement that lets users match a search
   pattern at the beginning of a file name.  With
   `dired-isearch-filenames-regexp', for example, using "`\f" or "^f"
   does not match a file name that begins with "f".  It's not obvious
   how to match the beginning of a file name.



In GNU Emacs 26.0.90 (build 3, x86_64-w64-mingw32)
 of 2017-10-13
Repository revision: 906224eba147bdfc0514090064e8e8f53160f1d4
Windowing system distributor `Microsoft Corp.', version 6.1.7601
Configured using:
 `configure --without-dbus --host=x86_64-w64-mingw32
 --without-compress-install 'CFLAGS=-O2 -static -g3''