don't check for changed files when checking out a new branch

This should never cause files to change, and was failing because we were asking for the git diff between the current branch and one that did not yet exist

src/model.ts

@@ -384,15 +384,17 @@ export class GitExtension implements IGitExtension {
       'git:checkout',
       async () => {
         let changes;
-        if (body.checkout_branch) {
-          changes = await this._changedFiles(
-            this._currentBranch.name,
-            body.branchname
-          );
-        } else if (body.filename) {
-          changes = { files: [body.filename] };
-        } else {
-          changes = await this._changedFiles('WORKING', 'HEAD');
+        if (!body.new_check) {
+          if (body.checkout_branch && !body.new_check) {
+            changes = await this._changedFiles(
+              this._currentBranch.name,
+              body.branchname
+            );
+          } else if (body.filename) {
+            changes = { files: [body.filename] };
+          } else {
+            changes = await this._changedFiles('WORKING', 'HEAD');
+          }
         }
 
         const d = await requestAPI<Git.ICheckoutResult>(
@@ -401,7 +403,7 @@ export class GitExtension implements IGitExtension {
           body
         );
 
-        changes.files?.forEach(file => this._revertFile(file));
+        changes?.files?.forEach(file => this._revertFile(file));
         return d;
       }
     );