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.

从 LibGit2Sharp.Tests/TestHelpers/BaseFixture.cs 中的 AssertRefLogEntry 以及 LibGit2Sharp.Tests/BranchFixture.cs 中失败的 CanCreateBranchFromCommit 测试开始。反复运行受影响的测试，检查所引用的 CI 失败，并比较预期的 reflog 时间戳与记录的时间戳。当受影响的测试在 CI 或本地不再间歇性失败时，即表示完成。