fix(git-node): prepare_release resilient checkout

In `git node release --prepare` when checking out a new proposal branch
make sure it's also possible to update an already existing branch of
that same name instead of failing and terminating the automation.

lib/prepare_release.js

@@ -93,9 +93,10 @@ export default class ReleasePreparation extends Session {
     } = this;
 
     // Create new proposal branch.
-    cli.startSpinner(`Creating new proposal branch for ${newVersion}`);
-    const proposalBranch = await this.createProposalBranch(releaseBranch);
-    cli.stopSpinner(`Created new proposal branch for ${newVersion}`);
+    cli.startSpinner(`Switching to proposal branch for ${newVersion}`);
+    const { proposalBranch, resultMsg } =
+      await this.createProposalBranch(releaseBranch);
+    cli.stopSpinner(resultMsg);
 
     const success = await this.cherryPickSecurityPRs(filterLabels);
     if (!success) {
@@ -201,9 +202,10 @@ export default class ReleasePreparation extends Session {
     }
 
     // Create new proposal branch.
-    cli.startSpinner(`Creating new proposal branch for ${newVersion}`);
-    await this.createProposalBranch();
-    cli.stopSpinner(`Created new proposal branch for ${newVersion}`);
+    cli.startSpinner(`Switching to proposal branch for ${newVersion}`);
+    const { resultMsg } =
+      await this.createProposalBranch();
+    cli.stopSpinner(resultMsg);
 
     if (this.isLTSTransition) {
       // For releases transitioning into LTS, fetch the new code name.
@@ -533,13 +535,37 @@ export default class ReleasePreparation extends Session {
     const { newVersion } = this;
     const proposalBranch = `v${newVersion}-proposal`;
 
-    await runAsync('git', [
-      'checkout',
-      '-b',
+    try {
+      await forceRunAsync('git', [
+        'checkout',
+        '-b',
+        proposalBranch,
+        base
+      ], { captureStdout: true, captureStderr: true, ignoreFailure: false });
+    } catch (err) {
+      const branchExistsRE = /fatal: a branch named '.*' already exists/i;
+      if (branchExistsRE.test(err.stderr)) {
+        await runAsync('git', [
+          'checkout',
+          proposalBranch
+        ]);
+        await runAsync('git', [
+          'reset',
+          '--hard',
+          base
+        ]);
+        return {
+          proposalBranch,
+          resultMsg: `Updated proposal branch for ${newVersion}`
+        };
+      } else {
+        throw err;
+      }
+    }
+    return {
       proposalBranch,
-      base
-    ]);
-    return proposalBranch;
+      resultMsg: `Created new proposal branch for ${newVersion}`
+    };
   }
 
   async updateNodeVersion() {