make git-annex_$version tags in the downloads repo

make git-annex_$version tags in the downloads repo

Note the use of git tag -f, this way if it takes several tries to get a
release published, it will use the final one for the tag.

Tags have been retrospectively added for past releases, using this:

for v in $(git log --oneline --grep="publishing git-annex "| sed 's/ publishing git-annex / /'); do foo=$(echo "$v" | sed 's/ .*//'); bar=$(echo "$v" | sed 's/.* //');  git tag git-annex_"$bar" "$foo" ; done