Allow building from a detached tag, since there is no need to calculate the increment

[gerwinjansen]

Description

Rejection of a detached head now only happens if no tag is available

Related Issue

Fixes #2301 GitVersionTask fails when building from a tag

Motivation and Context

Detached HEADs were always rejected, because a branch is needed to calculate the next version. But this is not required for tagged commits, since the version is directly derived from the tag.

How Has This Been Tested?

git init
git commit --allow-empty -m "init"
git branch task1
git tag v0.2
git checkout v0.2
gitversion

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.

[lucaritossa]

@asbjornu please give this PR an high priority so it can be released asap. The issue is very annoying...

[asbjornu]

This is most excellent. Thank you so much for fixing this, @gerwinjansen! 🙏