Turn branch name into label instead of metadata if no label exists
Turn branch name into label instead of metadata if no label exists

Only for non-master branches, having the branch name as a prerelease
label makes it easier to trivially dogfood a specific feature branch,
without having to tweak GitInfo.txt across branches.

So, for example, `dev` branch will get version numbers like 3.1.0-dev.5+sha.asdfasdf.

Branching for a feature `foo`, you'd automatically get version numbers like
3.1.0-foo.6+sha.asfdasdf.

If, however, we already had a branch that's creating an already labeled
release, such as 3.2.0-alpha, the version in (say) dev for that would be
3.2.0-alpha.15+dev.sha.asdfasdf. This allows you to tell apart a build for the
alpha that's being contributed by a specific branch.

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