build: add git hash

This commit makes the git hash ripgrep was built with available for use
in the version string.

We also do a few minor touchups in build.rs and src/app.rs.

build.rs

@@ -5,6 +5,7 @@ extern crate lazy_static;
 
 use std::env;
 use std::fs;
+use std::process;
 
 use clap::Shell;
 
@@ -13,14 +14,34 @@ use clap::Shell;
 mod app;
 
 fn main() {
+    // OUT_DIR is set by Cargo and it's where any additional build artifacts
+    // are written.
     let outdir = match env::var_os("OUT_DIR") {
-        None => return,
         Some(outdir) => outdir,
+        None => {
+            eprintln!(
+                "OUT_DIR environment variable not defined. \
+                 Please file a bug: \
+                 https://github.com/BurntSushi/ripgrep/issues/new");
+            process::exit(1);
+        }
     };
     fs::create_dir_all(&outdir).unwrap();
 
+    // Use clap to build completion files.
     let mut app = app::app();
     app.gen_completions("rg", Shell::Bash, &outdir);
     app.gen_completions("rg", Shell::Fish, &outdir);
     app.gen_completions("rg", Shell::PowerShell, &outdir);
+    // Note that we do not use clap's support for zsh. Instead, zsh completions
+    // are manually maintained in `complete/_rg`.
+
+    // Make the current git hash available to the build.
+    let result = process::Command::new("git")
+        .args(&["rev-parse", "--short=10", "HEAD"])
+        .output();
+    if let Ok(output) = result {
+        let hash = String::from_utf8_lossy(&output.stdout);
+        println!("cargo:rustc-env=RIPGREP_BUILD_GIT_HASH={}", hash);
+    }
 }