gen_version_info fails if no branch is checked out

[GigaGrunch]

When just checking out a commit (or tag like v0.2.1) and building flow, the version info is completely missing in the build. I believe that the reason is the usage of git log --pretty=oneline @{u}... because @{u} tries to resolve the remote tracking branch. Running the git command directly prints fatal: HEAD does not point to a branch.