bash: Support new 'git fetch' options

Support the new options --all, --prune, and --dry-run for
'git fetch'.

As the --multiple option was primarily introduced to enable
'git remote update' to be re-implemented in terms of 'git fetch'
(16679e37) and is not likely to be used much from the command
line, it does not seems worthwhile to complicate the code
(to support completion of multiple remotes) to handle it.

Signed-off-by: Björn Gustavsson <bgustavsson@gmail.com>
Acked-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>