Add commit hash to version command (#2312)

* Add commit to version command

* Replace unwrap with expect.

crates/nu-cli/build.rs

@@ -0,0 +1,41 @@
+use std::path::Path;
+use std::{env, fs, io};
+
+use git2::Repository;
+
+#[derive(Debug)]
+enum Error {
+    IoError(io::Error),
+    GitError(git2::Error),
+}
+
+impl From<git2::Error> for Error {
+    fn from(git_error: git2::Error) -> Self {
+        Self::GitError(git_error)
+    }
+}
+
+impl From<io::Error> for Error {
+    fn from(io_error: io::Error) -> Self {
+        Self::IoError(io_error)
+    }
+}
+
+fn main() -> Result<(), Error> {
+    let out_dir = env::var_os("OUT_DIR").expect(
+    "\
+        OUT_DIR environment variable not found. \
+        OUT_DIR is guaranteed to to exist in a build script by cargo - see \
+        https://doc.rust-lang.org/cargo/reference/environment-variables.html#environment-variables-cargo-sets-for-build-scripts\
+    ");
+    let latest_commit_hash = Repository::discover(env::current_dir()?)?
+        .head()?
+        .peel_to_commit()?
+        .id()
+        .to_string();
+
+    let commit_hash_path = Path::new(&out_dir).join("git_commit_hash");
+    fs::write(commit_hash_path, latest_commit_hash)?;
+
+    Ok(())
+}