feat(users/Profpatsch): add git-db

WIP: currently just a simple setup that creates an empty git repo if
it doesn’t exist yet, and writes a commit to it.

A simple database backed by a bare git repository.

WIP: Will speak a simple interactive protocol to query files and
update them atomically.

It could be made atomic on the git repo level, if a lock is taken
between reading the current commit ref and creating the commit.

Change-Id: I1fd30a046ac977063c3e08c36d96e835b35ff07d
Reviewed-on: https://cl.tvl.fyi/c/depot/+/3046
Tested-by: BuildkiteCI
Reviewed-by: Profpatsch <mail@profpatsch.de>

users/Profpatsch/git-db/default.nix

@@ -0,0 +1,8 @@
+{ depot, pkgs, lib, ... }:
+
+depot.nix.writers.rustSimple {
+  name = "git-db";
+  dependencies = [
+    depot.third_party.rust-crates.git2
+  ];
+} (builtins.readFile ./git-db.rs)