feat(shorebird_cli): add fetch to Git

packages/shorebird_cli/lib/src/git.dart

@@ -69,6 +69,24 @@ class Git {
     }
   }
 
+  /// Fetch branches/tags from the repository at [directory].
+  Future<void> fetch({required String directory, List<String>? args}) async {
+    final arguments = ['fetch', ...?args];
+    final result = await process.run(
+      executable,
+      arguments,
+      workingDirectory: directory,
+    );
+    if (result.exitCode != 0) {
+      throw ProcessException(
+        executable,
+        arguments,
+        '${result.stderr}',
+        result.exitCode,
+      );
+    }
+  }
+
   /// Returns the revision of the git repository located at [directory].
   Future<String> revParse({
     required String revision,

packages/shorebird_cli/test/src/git_test.dart

@@ -141,6 +141,54 @@ void main() {
       });
     });
 
+    group('fetch', () {
+      const directory = 'repository';
+      test('executes correct command', () async {
+        await expectLater(
+          runWithOverrides(
+            () => git.fetch(directory: directory),
+          ),
+          completes,
+        );
+        verify(
+          () => process.run(
+            'git',
+            ['fetch'],
+            workingDirectory: directory,
+          ),
+        ).called(1);
+      });
+
+      test('executes correct command w/args', () async {
+        final args = ['--tags'];
+        await expectLater(
+          runWithOverrides(
+            () => git.fetch(directory: directory, args: args),
+          ),
+          completes,
+        );
+        verify(
+          () => process.run(
+            'git',
+            ['fetch', ...args],
+            workingDirectory: directory,
+          ),
+        ).called(1);
+      });
+
+      test('throws ProcessException if process exits with error', () async {
+        const error = 'oops';
+        when(() => processResult.exitCode).thenReturn(ExitCode.software.code);
+        when(() => processResult.stderr).thenReturn(error);
+        expect(
+          () => runWithOverrides(() => git.fetch(directory: directory)),
+          throwsA(
+            isA<ProcessException>().having((e) => e.message, 'message', error),
+          ),
+        );
+      });
+    });
+
     group('revParse', () {
       const directory = './output';
       const revision = 'revision';