fix(backport): checkout from staging branch and ask to update if needed

lib/backport_session.js

@@ -61,7 +61,7 @@ export default class BackportSession extends Session {
         cli.stopSpinner(`${file} does not exist in current working tree`,
           cli.SPINNER_STATUS.WARN);
         continue;
-      };
+      }
       if (ancestors.length === 0) {
         cli.stopSpinner(`Cannot find ancestor commits of ${file}`,
           cli.SPINNER_STATUS.INFO);
@@ -155,12 +155,33 @@ export default class BackportSession extends Session {
       cli.log(`    - ${commit.sha} ${commit.title}`);
     }
 
+    if (!this.isLocalBranchExists(this.stagingBranch)) {
+      const shouldCreateStagingBranch = await cli.prompt(
+        `It seems like ${this.stagingBranch} is missing locally, ` +
+        'do you want to create it locally to get ready for backporting?', {
+          defaultAnswer: true
+        });
+
+      if (shouldCreateStagingBranch) {
+        this.syncBranchWithUpstream(this.stagingBranch);
+      }
+    } else if (!this.isBranchUpToDateWithUpstream(this.stagingBranch)) {
+      const shouldSyncBranch = await cli.prompt(
+        `It seems like your ${this.stagingBranch} is behind the ${this.upstream} remote` +
+        'do you want to sync it?', { defaultAnswer: true });
+
+      if (shouldSyncBranch) {
+        this.syncBranchWithUpstream(this.stagingBranch);
+      }
+    }
+
     const newBranch = `backport-${this.prid}-to-${this.target}`;
     const shouldCheckout = await cli.prompt(
       `Do you want to checkout to a new branch \`${newBranch}\`` +
       ' to start backporting?', { defaultAnswer: false });
+
     if (shouldCheckout) {
-      await runAsync('git', ['checkout', '-b', newBranch]);
+      await runAsync('git', ['checkout', '-b', newBranch, this.stagingBranch]);
     }
 
     const shouldAnalyze = await cli.prompt(
@@ -226,4 +247,56 @@ export default class BackportSession extends Session {
       };
     });
   }
+
+  getCurrentBranch() {
+    return runSync('git',
+      ['rev-parse', '--abbrev-ref', 'HEAD']
+    ).trim();
+  }
+
+  updateUpstreamRefs(branchName) {
+    runSync('git',
+      ['fetch', this.upstream, branchName]
+    );
+  }
+
+  getBranchCommit(branch) {
+    return runSync('git',
+      ['rev-parse', branch]
+    ).trim();
+  }
+
+  isBranchUpToDateWithUpstream(branch) {
+    this.updateUpstreamRefs(branch);
+
+    const localCommit = this.getBranchCommit(branch);
+    const upstreamCommit = this.getBranchCommit(`${this.upstream}/${branch}`);
+
+    return localCommit === upstreamCommit;
+  };
+
+  isLocalBranchExists(branch) {
+    try {
+      // will exit with code 1 if branch does not exist
+      runSync('git',
+        ['rev-parse', '--verify', '--quiet', branch]
+      );
+      return true;
+    } catch (e) {
+      return false;
+    }
+  }
+
+  syncBranchWithUpstream(branch) {
+    const currentBranch = this.getCurrentBranch();
+
+    runSync('git',
+      [
+        currentBranch !== branch ? 'fetch' : 'pull',
+        this.upstream,
+        `${branch}:${branch}`,
+        '-f'
+      ]
+    );
+  }
 }