fetch: allow command line --tags to override config

fetch: allow command line --tags to override config

Originally, if remote.<name>.tagopt was set, the --tags and option would
have no effect when given to git fetch. So if
tagopt="--no-tags"

	git fetch --tags

would not actually fetch tags.

This patch changes this behavior to only follow what is written in the
config if there is no option passed by the command line.

Signed-off-by: Daniel Johnson <ComputerDruid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>