[git-branch] Improve types (#31676)

types/git-branch/git-branch-tests.ts

@@ -1,14 +1,18 @@
 import gitBranch = require('git-branch');
 
 gitBranch()
-  .then((name) => {});
+  .then((name) => {
+      name; // $ExpectType string
+  });
 
 gitBranch('cwd')
-  .then((name) => {});
+  .then((name) => {
+      name; // $ExpectType string
+  });
 
-gitBranch.sync();
-gitBranch.sync('cwd');
+gitBranch.sync(); // $ExpectType string
+gitBranch.sync('cwd');  // $ExpectType string
 
-gitBranch((err, name) => {});
+gitBranch((err, name) => {}); // $ExpectType void
 
-gitBranch('cwd', (err, name) => {});
+gitBranch('cwd', (err, name) => {}); // $ExpectType void

types/git-branch/index.d.ts

@@ -8,7 +8,7 @@ export = GitBranch;
 
 declare function GitBranch(cwd?: string): Promise<string>;
 declare function GitBranch(cwd?: string, callback?: (err: null | string, name: string) => void): void;
-declare function GitBranch(callback?: (err: null | string, name: string) => void): void;
+declare function GitBranch(callback: (err: null | string, name: string) => void): void;
 
 declare namespace GitBranch {
   function sync(cwd?: string): string;