Teach fetch --dry-run as users of "git remote prune" switching to "git fetch
--prune" may expect it. Unfortunately OPT__DRY_RUN() cannot be used as fetch
already uses "-n" for something else.
Signed-off-by: Jay Soffian <jaysoffian@gmail.com> Signed-off-by: Björn Gustavsson <bgustavsson@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>