fetch tags

[extrawurst]

thanks to @cruessler and following #483 we now have two nice to have features left:

• explicit fetch-tags command repo wide (right now I think we only fetch them for the current branch)
• visualise tags in the list with an arrow icon if a tag is pushable (its not present on remote, we have that buried already in the push-tags-feature) (done in Show symbol if tag is not yet pushed #776)

those can be tackled individually of course