bug#8795: 24.0.50; `completion-try-completion' addition of METADATA arg

[Drew Adams]

1. The addition of arg METADATA, without making it optional, breaks
existing code that calls the function with only the first four args.
 
What value of METADATA should be passed to get the equivalent of the
behavior prior to the addition of this arg?  nil?
 
How about making this arg optional, so user code does not need to
special-case Emacs 24 wrt Emacs 23?


2. The METADATA arg is not even described in the doc string.
Users have no way to know how to fix the code this breaks.
Please describe the arg in the doc string.

In GNU Emacs 24.0.50.1 (i386-mingw-nt5.1.2600)
 of 2011-05-31 on 3249CTO
Windowing system distributor `Microsoft Corp.', version 5.1.2600
configured using `configure --with-gcc (4.5) --no-opt --cflags
-Ic:/build/include'