Configure structopt to report the right version

By default, structopt reports the value provided by
the env var CARGO_PKG_VERSION, feeded by Cargo when reading
Cargo.toml. However for Garage we use a versioning based on git,
so we often report a version that is behind the real version.
In this commit, we create garage_util::version::garage() that
reports the right version and configure all structopt subcommands
to call this function instead of using the env var.

src/util/version.rs

@@ -0,0 +1,7 @@
+pub fn garage() -> &'static str {
+	option_env!("GIT_VERSION").unwrap_or(git_version::git_version!(
+		prefix = "git:",
+		cargo_prefix = "cargo:",
+		fallback = "unknown"
+	))
+}