Various fixes for GH-2967

[asbjornu]

This PR should fix #2967.

Description

There are quite a bit of assumptions in the Mainline codepath that doesn't seem to hold true, such as all branches having a merge base. I believe it should be a viable fallback to look for the current branch's origin commit when a merge base cannot be found. That and a few other fixes (mostly null protection) is added to fix #2967.

Related Issue

Fixes #2967.

Motivation and Context

We don't want GitVersion to throw NullReferenceException. Ever.

How Has This Been Tested?

I have not been able to conjure a test that provokes this problem yet, but I'll try once I find time. I'm keeping this in draft until such a test is implemented.

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

[asbjornu]

As I expected, I'm unable to reproduce #2967 in a local test. The following attempted reproduction fails on AssertFullSemver, not by throwing a NullReferenceException:

[Test]
public void BranchWithoutMergeBaseMainlineBranchIsFound()
{
    var currentConfig = new Config
    {
        VersioningMode = VersioningMode.Mainline,
        AssemblyFileVersioningScheme = AssemblyFileVersioningScheme.MajorMinorPatchTag
    };

    using var fixture = new EmptyRepositoryFixture();
    fixture.Repository.MakeACommit();
    Commands.Checkout(fixture.Repository, fixture.Repository.CreateBranch("master"));
    fixture.Repository.Branches.Remove(fixture.Repository.Branches["main"]);
    fixture.Repository.MakeCommits(2);
    Commands.Checkout(fixture.Repository, fixture.Repository.CreateBranch("issue-branch"));
    fixture.Repository.MakeACommit();
    fixture.AssertFullSemver("0.1.3-issue-branch.1", currentConfig);
}

I'm adding the test nonetheless, but it's a bit unfortunate that we are unable to reproduce the NullReferenceException beforehand.

[mergify]

Thank you @asbjornu for your contribution!