Setting tracking branch fails with "ref 'refs/remotes/origin/master' doesn't match the destination"

Reproduction steps

• a repo with multiple overlapping fetch specs:

[remote "origin"]
	url = ../multifetchspec_testrepo.git/
	fetch = +refs/pull/28/head:refs/remotes/origin/PR-28
	fetch = +refs/heads/master:refs/remotes/origin/master
	fetch = +refs/heads/*:refs/remotes/origin/*

• repo doesn't have local master branch (but remote does)
• call repo.Branches.Update("master", b => b.TrackedBranch = trackedBranch.CanonicalName); (with trackedBranch pointing to refs/remotes/origin/master).

Expected behavior

Tracking branch gets set.

Actual behavior

Exception:

LibGit2Sharp.LibGit2SharpException: ref 'refs/remotes/origin/master' doesn't match the destination

LibGit2Sharp.LibGit2SharpException
ref 'refs/remotes/origin/master' doesn't match the destination
   at LibGit2Sharp.Core.Ensure.HandleError(Int32 result) in D:\Develop\libgit2sharp\LibGit2Sharp\Core\Ensure.cs:line 154
   at LibGit2Sharp.Core.Ensure.ZeroResult(Int32 result) in D:\Develop\libgit2sharp\LibGit2Sharp\Core\Ensure.cs:line 172
   at LibGit2Sharp.Core.Proxy.git_refspec_rtransform(IntPtr refSpecPtr, String name) in D:\Develop\libgit2sharp\LibGit2Sharp\Core\Proxy.cs:line 2085
   at LibGit2Sharp.Remote.FetchSpecTransformToSource(String reference) in D:\Develop\libgit2sharp\LibGit2Sharp\Remote.cs:line 138
   at LibGit2Sharp.BranchUpdater.GetUpstreamInformation(String canonicalName, String& remoteName, String& mergeBranchName) in D:\Develop\libgit2sharp\LibGit2Sharp\BranchUpdater.cs:line 188
   at LibGit2Sharp.BranchUpdater.SetUpstream(String upstreamBranchName) in D:\Develop\libgit2sharp\LibGit2Sharp\BranchUpdater.cs:line 116
   at LibGit2Sharp.BranchUpdater.set_TrackedBranch(String value) in D:\Develop\libgit2sharp\LibGit2Sharp\BranchUpdater.cs:line 53
   at LibGit2Sharp.Tests.BranchFixture.<>c__DisplayClass43_0.<CanSetTrackedBranch_MultipleFetchSpecs>b__0(BranchUpdater b) in D:\Develop\libgit2sharp\LibGit2Sharp.Tests\BranchFixture.cs:line 898
   at LibGit2Sharp.BranchCollection.Update(Branch branch, Action`1[] actions) in D:\Develop\libgit2sharp\LibGit2Sharp\BranchCollection.cs:line 292

Version of LibGit2Sharp (release number or SHA1)

4de700fedf87a5c9de10b921a636cc2f21d8964d

Operating system(s) tested; .NET runtime tested

Windows 10, Ubuntu 18.04

从 LibGit2Sharp.Tests/BranchFixture.cs 中失败的测试 CanSetTrackedBranch_MultipleFetchSpecs 开始。使用复现中的重叠 fetch 规范，跟踪 BranchUpdater.GetUpstreamInformation 和 Remote.FetchSpecTransformToSource。当设置跟踪分支成功，并且回归测试在所述场景中通过时，该问题就完成了。