super annoying completion change

[Miles Bader]

I notice that recently the behavior of bash completion w/r/t
environment variables has changed, in a reallllly annoying way:

If one has an environment variable like:

   ls=/usr/local/src

and then hit <TAB> to complete the following command line:

   $ cd $ls/ema<TAB>

where the intent is to do "cd /usr/local/src/emacs", then:

* The _old_ behavior of bash was to (1) expand "$ls" to
  "/usr/local/src", and then (2) proceed to complete
  "/usr/local/src/ema" into "/usr/local/src/emacs".  Handy!

* The _new_ behavior is to just add a backslash before "$" and then
  (apparently) try to complete on the literal-$-prefixed string --
  which of course usually fails, as there is no such subdirectory.
  Gahhhh!

Given that filenames/directories containing a literal "$" are quite
rare in general, and the old behavior enabled a _very_ useful way of
using environment variables as "bookmarks", I think this change should
be reverted (obviously compatibility is not a concern, as this change
was itself an incompatible change).

My bash version is "4.2.20(1)-release (x86_64-pc-linux-gnu)" in Debian
sid.  If this isn't actually a bash problem then apologies... (dunno
what it would be then, though)!

Thanks,

-Miles

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