bash-completion: Try bash completions before simple filetype

bash-completion: Try bash completions before simple filetype

When a git completion is not found, a bash shell should try bash-type
completions first before going to standard filetype completions. This
patch adds "-o bashdefault" to the completion line. If that option is
not available, it uses the old method.

This behavior was inspired by Mercurial's bash completion script.

Signed-off-by: Ted Pavlic <ted@tedpavlic.com>
Acked-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>