[Bug] next-version on Mainline

[td0wn]

Describe the bug
Setting next-version prevents Mainline mode from counting direct & merge commits when computing the version, since the changes in commit c701080, I believe.

Expected Behavior

The next-version setting should affect only the baseline version and the commit traversal in Mainline mode should still be executed, as was the previous behavior.

Actual Behavior

No commits are traversed/recognized as bumping the version so the next-version is just used as the computed version.

Prior to this change the next-version setting was used just for determining the baseline version, but now that setting is being consulted for commit traversal in Mainline mode and resulting in different behavior. I don't think this was intentional?

Possible Fix

Don't check for next-version being null/empty when deciding whether to walk the commit graph in Mainline mode. Basically I don't think this conditional check should be there: https://github.com/GitTools/GitVersion/blob/main/src/GitVersion.Core/VersionCalculation/MainlineVersionCalculator.cs#L55

Context

Computed version when in Mainline mode and setting next-version results in different values when upgrading to versions released after c701080 merged.

Your Environment

• Version Used: 5.6.4
• Operating System and version (Windows 10, Ubuntu 18.04): Windows 10 & Ubuntu 20.04

[asbjornu]

Duplicate of #2441. Please remove next-version and add a git tag with the same version number, on the appropriate commit.