Include git HEAD rev in version output

Closes #241

cli/build.rs

@@ -0,0 +1,34 @@
+use std::process::Command;
+
+fn main() {
+    // Query HEAD revision and expose as $TC_GIT_REV during build
+    //
+    // Adapted from https://stackoverflow.com/questions/43753491
+    let cmd = Command::new("git")
+        .args(&["rev-parse", "--short", "HEAD"])
+        .spawn()
+        // Wait for process to exit
+        .and_then(|cmd| cmd.wait_with_output())
+        // Handle error if failed to launch git
+        .map_err(|_e| println!("cargo:warning=Failed to run 'git' to determine HEAD rev"))
+        // Remap to Some/None for simpler error handling
+        .ok()
+        // Handle command failing
+        .and_then(|o| {
+            if o.status.success() {
+                Some(o)
+            } else {
+                println!(
+                    "cargo:warning='git' exited with non-zero exit code while determining HEAD rev"
+                );
+                None
+            }
+        })
+        // Get output as UTF-8 string
+        .map(|out| String::from_utf8(out.stdout).expect("Invalid output in stdout"));
+
+    // Only output git rev if successful
+    if let Some(h) = cmd {
+        println!("cargo:rustc-env=TC_GIT_REV={}", h);
+    }
+}