Use read_to_string.

Author: Junfeng Li

build.rs

@@ -1,17 +1,9 @@
-use std::process::Command;
-
-fn git_hash() -> String {
-    let output = Command::new("cat")
-        .args(&[".git/refs/heads/next"])
-        .output()
-        .unwrap();
-    String::from_utf8_lossy(&output.stdout).into()
-}
+use std::fs::read_to_string;
 
 fn main() {
     let git_hash = option_env!("TRAVIS_COMMIT")
         .map(|s| s.into())
-        .unwrap_or_else(|| git_hash());
+        .unwrap_or_else(|| read_to_string(".git/refs/heads/next").unwrap_or_default());
 
     println!("cargo:rustc-env=GIT_HASH={}", git_hash);
 }