Add support for git flow using git-flow-next

git-flow-next (https://github.com/gittower/git-flow-next) uses a
different config schema than legacy git-flow:
gitflow.branch.<type>.prefix instead of gitflow.prefix.<type>.
Recognize both schemas in GetGitFlowPrefixMap by querying each and
merging into a single prefix → branchType map. GitFlowEnabled now
consults the merged map so a next-only setup counts as enabled.

When both schemas configure the same prefix, the legacy entry wins.
In normal usage both schemas agree, so the rule mainly matters as a
deterministic tie-breaker.