This updates dovecot-trees to 2.1.0.

It also changes the -fetch method to use file instead of git. The
problem is I'm not sure if I picked the correct link.

https://0xacab.org/riseuplabs/trees/tree/v2.1.0 is the url I used, in
the webbrowser I selected the dropdown menu in their gitlab to save the
master. I suppose that v2.1.0 is a tag and not just a branch, and that
this downloads the tag at this commit.
If you know a better way, or know this doesn't work, I appreciate
feedback. Maybe we could document how to get this from gitlab based git
instances, so that it's useful for packagers.