cli: rewrite get_git_hash_from_jj in a functional style

Author: ipetkov

cli/build.rs

@@ -57,7 +57,7 @@ fn get_git_hash_from_nix() -> Option<String> {
 }
 
 fn get_git_hash_from_jj() -> Option<String> {
-    if let Ok(output) = Command::new("jj")
+    Command::new("jj")
         .args([
             "--ignore-working-copy",
             "--color=never",
@@ -67,17 +67,15 @@ fn get_git_hash_from_jj() -> Option<String> {
             "-T=commit_id ++ '-'",
         ])
         .output()
-    {
-        if output.status.success() {
+        .ok()
+        .filter(|output| output.status.success())
+        .map(|output| {
             let mut parent_commits = String::from_utf8(output.stdout).unwrap();
             // If a development version of `jj` is compiled at a merge commit, this will
             // result in several commit ids separated by `-`s.
             parent_commits.truncate(parent_commits.trim_end_matches('-').len());
-            return Some(parent_commits);
-        }
-    }
-
-    None
+            parent_commits
+        })
 }
 
 fn get_git_hash_from_git() -> Option<String> {