fix: respect existing trailers in commit messages

lib/landing_session.js

@@ -306,9 +306,24 @@ export default class LandingSession extends Session {
     const original = runSync('git', [
       'show', 'HEAD', '-s', '--format=%B'
     ]).trim();
+
+    // git has very specific rules about what is a trailer and what is not.
+    // Instead of trying to implement those ourselves, let git parse the
+    // original commit message and see if it outputs any trailers.
+    const originalHasTrailers = runSync('git', [
+      'interpret-trailers', '--parse', '--no-divider'
+    ], {
+      input: `${original}\n`
+    }).trim().length !== 0;
+
     const metadata = this.metadata.trim().split('\n');
     const amended = original.split('\n');
-    if (amended[amended.length - 1] !== '') {
+
+    // If the original commit message already contains trailers (such as
+    // "Co-authored-by"), we simply add our own metadata after those. Otherwise,
+    // we have to add an empty line so that git recognizes our own metadata as
+    // trailers in the amended commit message.
+    if (!originalHasTrailers) {
       amended.push('');
     }
 
@@ -317,9 +332,13 @@ export default class LandingSession extends Session {
     const REVIEW_RE = /Reviewed-By\s*:\s*(\S+)/i;
 
     for (const line of metadata) {
-      if (original.includes(line)) {
-        if (line) {
+      if (line.length !== 0 && original.includes(line)) {
+        if (originalHasTrailers) {
           cli.warn(`Found ${line}, skipping..`);
+        } else {
+          cli.error('Git found no trailers in the original commit message, ' +
+                    `but '${line}' is present and should be a trailer.`);
+          process.exit(1);  // make it work with git rebase -x
         }
       } else {
         if (line.match(BACKPORT_RE)) {