bug#15749: 24.3.50; reduced font size for *Backtrace* is lost for each step

[Drew Adams <drew.adams@oracle.com>]

emacs -Q

Use `debug-on-entry' for some function, invoke that function, and try to
step through the debugger.

As soon as buffer *Backtrace* is created, use `S-mouse-1' and choose
`Decrease Buffer Text Size'.  OK.

Now step, using `d'.  At the first step, buffer *Backtrace* returns to
the original size text.  This font-size change is supposed to be per
buffer, no matter where the buffer is displayed.

Presumably because Emacs, behind the scene, kills and re-creates buffer
`*Backtrace*', the user-expected behavior is broken.  The debugger
should remember the user-chosen font size and reinstate it when it does
its otherwise user-invisible, behind-the-scene buffer manipulations.

In fact, every user-pertinent setting in effect for buffer *Backtrace*
should be restored, each time the buffer is "updated"/re-created.



In GNU Emacs 24.3.50.1 (i686-pc-mingw32)
 of 2013-10-19 on LEG570
Bzr revision: 114715 rgm@gnu.org-20131019023520-s8mwtib7xcx9e05w
Windowing system distributor `Microsoft Corp.', version 6.1.7601
Configured using:
 `configure --enable-checking 'CFLAGS=-O0 -g3' CPPFLAGS=-DGLYPH_DEBUG=1'