Use `--tags` option when fetching repo updates

The default is to fetch updates from a repo instead of cloning the complete repo again, which is great.

The drawback is that then tags are usually not available and using things like git describe in build/packaging/deploy steps won't correctly work any more.

So I propose to add the --tags option to the git fetch command (writeFetchCmd in shells/bash.go, etc.) so that tag objects (annotated tags) are also fetched. See https://git-scm.com/docs/git-fetch

That should make git describe (and anything else that need the tags) work with the faster (and default) fetch option.