git: add 'UpdateBareRepo()' mock

The 'UpdateBareRepo()' method was added to the 'GitHelper' interface in
62f35ce (bundles: fetch changes before building incremental bundle,
2023-03-14), but the corresponding mock method was not added to
'MockGitHelper'. Add it so that 'MockGitHelper' can be used as a 'GitHelper'
mock in later commits.

Signed-off-by: Victoria Dye <vdye@github.com>

Author: vdye

internal/testhelpers/mocks.go

@@ -224,3 +224,8 @@ func (m *MockGitHelper) CloneBareRepo(ctx context.Context, url string, destinati
 	fnArgs := m.Called(ctx, url, destination)
 	return fnArgs.Error(0)
 }
+
+func (m *MockGitHelper) UpdateBareRepo(ctx context.Context, repoDir string) error {
+	fnArgs := m.Called(ctx, repoDir)
+	return fnArgs.Error(0)
+}