Unstable tests based on AssertRefLogEntry

Several tests are unstable and fail all too frequently on CI and perhaps locally.

From this run of this commit the windows agent happened to fail with this log:

  X LibGit2Sharp.Tests.BranchFixture.CanCreateBranchFromCommit [63ms]
  Error Message:
   Assert.InRange() Failure
Range:  (1/31/2020 4:54:25 AM +00:00 - 1/31/2020 4:54:25 AM +00:00)
Actual: 1/31/2020 4:54:24 AM +00:00
  Stack Trace:
     at LibGit2Sharp.Tests.TestHelpers.BaseFixture.AssertRefLogEntry(IRepository repo, String canonicalName, String message, ObjectId from, ObjectId to, Identity committer, DateTimeOffset before) in D:\a\libgit2sharp\libgit2sharp\LibGit2Sharp.Tests\TestHelpers\BaseFixture.cs:line 471
   at LibGit2Sharp.Tests.BranchFixture.CanCreateBranchFromCommit() in D:\a\libgit2sharp\libgit2sharp\LibGit2Sharp.Tests\BranchFixture.cs:line 205

It looks to me like the before local variable is still set to a later value than is recorded in the commit at times, for whatever reason.

從 LibGit2Sharp.Tests/TestHelpers/BaseFixture.cs 中的 AssertRefLogEntry 以及 LibGit2Sharp.Tests/BranchFixture.cs 中失敗的 CanCreateBranchFromCommit 測試開始。反覆執行受影響的測試，檢查所引用的 CI 失敗，並比較預期的 reflog 時間戳與記錄的時間戳。當受影響的測試在 CI 或本機不再間歇性失敗時，即表示完成。