bug#17829: 24.4.50; `C-t' in empty minibuffer traps user in read-only text

[Drew Adams]

emacs -Q

`C-t' is `transpose-chars'. 

`M-x C-t' gives you a "Text is read-only" error, and it moves point
backward into the minibuffer prompt.  Repeating it does not move point
further backward but continues to raise the error.

Trying to type text then results in the same error message.  The user
can become confused, not realizing that point is in fact inside the
prompt area.  S?he can then use `C-f' to move back out of the prompt
area, but `C-t' should be smarter.

`transpose-chars' should presumably limit itself to field motion, or
should in some other way take such a situation into account.

In GNU Emacs 24.4.50.1 (i686-pc-mingw32)
 of 2014-06-17 on ODIEONE
Bzr revision: 117359 monnier@iro.umontreal.ca-20140617193358-2t1nl1te9gc2mqrx
Windowing system distributor `Microsoft Corp.', version 6.1.7601
Configured using:
 `configure --prefix=/c/Devel/emacs/snapshot/trunk
 --enable-checking=yes,glyphs 'CFLAGS=-O0 -g3'
 LDFLAGS=-Lc:/Devel/emacs/lib 'CPPFLAGS=-DGC_MCHECK=1
 -Ic:/Devel/emacs/include''