feat(git-node): add limited support for cross-repo PRs

lib/landing_session.js

@@ -82,12 +82,38 @@ export default class LandingSession extends Session {
   }
 
   async downloadAndPatch() {
-    const { cli, upstream, prid, expectedCommitShas } = this;
+    const { cli, upstream, prid, expectedCommitShas, crossRepoPR } = this;
+
+    if (crossRepoPR) {
+      const { owner, repo } = this;
+      cli.warn(`The configured remote "${upstream}" does not point to the ` +
+        'Pull Request repository.');
+      const isHeadAMergeCommit = () => {
+        try {
+          return runSync('git', ['rev-parse', 'FETCH_HEAD^2']).trim();
+        } catch {}
+      };
+      do {
+        cli.warn('Fetching commits from a different upstream might require user intervention.');
+
+        cli.info('Run the following command to fetch the commit(s):');
+        cli.info(`git fetch git@github.com:${owner}/${repo}.git refs/pull/${prid}/merge`);
+        const response = await cli.prompt('Should NCU run the command for you?',
+          { defaultAnswer: false });
+        if (response) {
+          cli.startSpinner(`Downloading patch for ${prid}`);
+          await forceRunAsync('git',
+            ['fetch', `git@github.com:${owner}/${repo}.git`, `refs/pull/${prid}/merge`],
+            { ignoreFailure: false });
+        }
+      } while (!isHeadAMergeCommit());
+    } else {
+      cli.startSpinner(`Downloading patch for ${prid}`);
+      await runAsync('git', [
+        'fetch', upstream,
+        `refs/pull/${prid}/merge`]);
+    }
 
-    cli.startSpinner(`Downloading patch for ${prid}`);
-    await runAsync('git', [
-      'fetch', upstream,
-      `refs/pull/${prid}/merge`]);
     // We fetched the commit that would result if we used `git merge`.
     // ^1 and ^2 refer to the PR base and the PR head, respectively.
     const [base, head] = await runAsync('git',
@@ -472,7 +498,7 @@ export default class LandingSession extends Session {
     }
 
     cli.separator();
-    cli.log('The following commits are ready to be pushed to ' +
+    cli.log('The following commits are landable on ' +
       `${upstream}/${branch}`);
     cli.log(`- ${strayVerbose.join('\n- ')}`);
     cli.separator();
@@ -483,8 +509,15 @@ export default class LandingSession extends Session {
       willBeLanded = `${head}...${willBeLanded}`;
     }
 
+    cli.startSpinner('Removing temporary files');
     this.cleanFiles();
-    cli.log('Temporary files removed.');
+    cli.stopSpinner('Temporary files removed.');
+    if (this.crossRepoPR) {
+      cli.warn(`The configured remote "${upstream}" does not point to the ` +
+          'Pull Request repository.');
+      cli.info(`${willBeLanded} should likely not be pushed to "${upstream}"`);
+      return;
+    }
     cli.log('To finish landing:');
     cli.log('1. Run: ');
     cli.log(`   git push ${upstream} ${branch}`);

lib/session.js

@@ -33,10 +33,11 @@ export default class Session {
       const upstreamHref = runSync('git', [
         'config', '--get',
       `remote.${upstream}.url`]).trim();
-      if (!new RegExp(`${owner}/${repo}(?:.git)?$`).test(upstreamHref)) {
+      if (!upstreamHref.endsWith(`${owner}/${repo}.git`) &&
+          !upstreamHref.endsWith(`${owner}/${repo}`)) {
         cli.warn('Remote repository URL does not point to the expected ' +
         `repository ${owner}/${repo}`);
-        cli.setExitCode(1);
+        this.crossRepoPR = true;
       }
     }
   }