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.

Author: ruyadorno

lib/prepare_release.js

@@ -523,12 +523,24 @@ export default class ReleasePreparation extends Session {
     const { newVersion } = this;
     const proposalBranch = `v${newVersion}-proposal`;
 
-    await runAsync('git', [
-      'checkout',
-      '-b',
-      proposalBranch,
-      base
-    ]);
+    try {
+      await forceRunAsync('git', [
+        'checkout',
+        '-b',
+        proposalBranch,
+        base
+      ], { captureStdout: true, captureStderr: true, ignoreFailures: false });
+    } catch (err) {
+      const branchExistsRE = /fatal: a branch named '.*' already exists/i;
+      if (branchExistsRE.test(err.stderr)) {
+        await runAsync('git', [
+          'checkout',
+          proposalBranch
+        ]);
+      } else {
+        throw err;
+      }
+    }
     return proposalBranch;
   }