filter git revision on git command success exit code

The unwrap (or '.ok()' rather) triggers for some errors but not
negative status codes. In the case where helix is being packaged
in an empty git repository, the existing mechanism will fail because

    git init
    git rev-parse HEAD

gives a negative exit code and prints to stderr

    stderr: "fatal: ambiguous argument 'HEAD': unknown revision or path not in the working tree....

with a stdout of "HEAD\n" (too short to slice with [..8]).

helix-term/build.rs

@@ -6,6 +6,7 @@ fn main() {
         .args(&["rev-parse", "HEAD"])
         .output()
         .ok()
+        .filter(|output| output.status.success())
         .and_then(|x| String::from_utf8(x.stdout).ok());
 
     let version: Cow<_> = match git_hash {