Show commit hash in version when built from source

Users might file an issue while running an older version, but the issue
has already been fixed in master. Adding the git revision to the version
output will expedite this diagnosis. For example:

    $ zoxide -V
    zoxide 0.3.0-0191eea

build.rs

@@ -0,0 +1,14 @@
+fn main() {
+    let mut hash = std::process::Command::new("git")
+        .args(&["rev-parse", "--short", "HEAD"])
+        .output()
+        .ok()
+        .and_then(|proc| String::from_utf8(proc.stdout).ok())
+        .unwrap_or_default();
+
+    if !hash.is_empty() {
+        hash = format!("-{}", hash);
+    }
+
+    println!("cargo:rustc-env=GIT_HASH={}", hash);
+}

src/main.rs

@@ -8,7 +8,10 @@ use anyhow::Result;
 use structopt::StructOpt;
 
 #[derive(Debug, StructOpt)]
-#[structopt(about = "A cd command that learns your habits")]
+#[structopt(
+    about = "A cd command that learns your habits",
+    version = concat!(env!("CARGO_PKG_VERSION"), env!("GIT_HASH"))
+)]
 enum Zoxide {
     Add(subcommand::Add),
     Import(subcommand::Import),