completion: add hack to enable file mode in bash < 4

completion: add hack to enable file mode in bash < 4

This way we don't need all the compat stuff, different filters, and so
on. Also, now we complete exactly the same in bash 3 and bash 4.

This is the way bash-completion did it for quite some time, when bash 3
was supported. For more information about the hack:

http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=272660#64

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>