tag: change default of `pager.tag` to "on"

tag: change default of `pager.tag` to "on"

The previous patch taught `git tag` to only respect `pager.tag` in
list-mode. That patch left the default value of `pager.tag` at "off".

After that patch, it makes sense to let the default value be "on"
instead, since it will help with listing many tags, but will not hurt
users of `git tag -a` as it would have before. Make that change. Update
documentation and tests.

Signed-off-by: Martin Ågren <redacted>
Signed-off-by: Junio C Hamano <redacted>