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.

LibGit2Sharp.Tests/TestHelpers/BaseFixture.cs の AssertRefLogEntry と、LibGit2Sharp.Tests/BranchFixture.cs の失敗している CanCreateBranchFromCommit テストから始めます。対象のテストを繰り返し実行し、参照されている CI の失敗を調査して、期待される reflog のタイムスタンプと記録されたタイムスタンプを比較します。対象のテストが CI 上でもローカルでも断続的に失敗しなくなれば完了です。