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

Bắt đầu với test đang thất bại CanSetTrackedBranch_MultipleFetchSpecs trong LibGit2Sharp.Tests/BranchFixture.cs. Truy vết BranchUpdater.GetUpstreamInformation và Remote.FetchSpecTransformToSource bằng các fetch specification chồng lấn trong bản tái hiện. Issue được hoàn tất khi việc thiết lập tracked branch thành công và regression test vượt qua trong các kịch bản đã nêu.