`azuredevops_git_repository` - Fix cannot set repo `default _branch` when updating

[xuzhang3]

All Submissions:

• [x] Have you added an explanation of what your changes do and why you'd like us to include them?
• [x] I have updated the documentation accordingly.
• [x] I have added tests to cover my changes.
• [x] All new and existing tests passed.
• [x] My code follows the code style of this project.
• [x] I ran lint checks locally prior to submission.
• [x] Have you checked to ensure there aren't other open PRs for the same update/change?

What about the current behavior has changed?

Issue Number: Fixes: #1016

Does this introduce a change to go.mod, go.sum or vendor/?

• [ ] Yes
• [x] No

Does this introduce a breaking change?

• [ ] Yes
• [x] No

Any relevant logs, error output, etc?

(If it’s long, please paste to https://ghostbin.com/ and insert the link here.)

Other information

[xuzhang3]

=== RUN   TestAccGitRepo_CreateAndUpdate
=== PAUSE TestAccGitRepo_CreateAndUpdate
=== RUN   TestAccGitRepo_Create_IncorrectInitialization
=== PAUSE TestAccGitRepo_Create_IncorrectInitialization
=== RUN   TestAccGitRepo_Create_Import
=== PAUSE TestAccGitRepo_Create_Import
=== RUN   TestAccGitRepo_RepoInitialization_Clean
=== PAUSE TestAccGitRepo_RepoInitialization_Clean
=== RUN   TestAccGitRepo_RepoInitialization_Uninitialized
=== PAUSE TestAccGitRepo_RepoInitialization_Uninitialized
=== RUN   TestAccGitRepo_RepoFork_BranchNotEmpty
=== PAUSE TestAccGitRepo_RepoFork_BranchNotEmpty
=== CONT  TestAccGitRepo_CreateAndUpdate
=== CONT  TestAccGitRepo_RepoInitialization_Clean
=== CONT  TestAccGitRepo_RepoFork_BranchNotEmpty
=== CONT  TestAccGitRepo_Create_Import
=== CONT  TestAccGitRepo_Create_IncorrectInitialization
=== CONT  TestAccGitRepo_RepoInitialization_Uninitialized
--- PASS: TestAccGitRepo_Create_IncorrectInitialization (3.91s)
--- PASS: TestAccGitRepo_RepoFork_BranchNotEmpty (48.91s)
--- PASS: TestAccGitRepo_RepoInitialization_Uninitialized (55.08s)
--- PASS: TestAccGitRepo_RepoInitialization_Clean (57.13s)
--- PASS: TestAccGitRepo_CreateAndUpdate (68.76s)
--- PASS: TestAccGitRepo_Create_Import (70.11s)
PASS
ok      github.com/microsoft/terraform-provider-azuredevops/azuredevops/internal/acceptancetests        77.680s