lint: Call lint_commit_msg from test_runner

Allowing to call the check from the test_runner allows for consistent
error messages. Also, manually setting the commit range is no longer
needed.

test/lint/test_runner/src/main.rs

@@ -73,6 +73,11 @@ fn get_linter_list() -> Vec<&'static Linter> {
             name: "scripted_diff",
             lint_fn: lint_scripted_diff
         },
+        &Linter {
+            description: "Check that commit messages have a new line before the body or no body at all.",
+            name: "commit_msg",
+            lint_fn: lint_commit_msg
+        },
         &Linter {
             description: "Check that tabs are not used as whitespace",
             name: "tabs_whitespace",
@@ -243,6 +248,42 @@ fn lint_scripted_diff() -> LintResult {
     }
 }
 
+fn lint_commit_msg() -> LintResult {
+    let mut good = true;
+    let commit_hashes = check_output(git().args(&[
+        "-c",
+        "log.showSignature=false",
+        "log",
+        &commit_range(),
+        "--format=%H",
+    ]))?;
+    for hash in commit_hashes.lines() {
+        let commit_info = check_output(git().args([
+            "-c",
+            "log.showSignature=false",
+            "log",
+            "--format=%B",
+            "-n",
+            "1",
+            hash,
+        ]))?;
+        if let Some(line) = commit_info.lines().nth(1) {
+            if !line.is_empty() {
+                println!(
+                        "The subject line of commit hash {} is followed by a non-empty line. Subject lines should always be followed by a blank line.",
+                        hash
+                    );
+                good = false;
+            }
+        }
+    }
+    if good {
+        Ok(())
+    } else {
+        Err("".to_string())
+    }
+}
+
 fn lint_py_lint() -> LintResult {
     let bin_name = "ruff";
     let checks = format!(