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.
Lift the inline parsing in FinishCmdObj into parseGitFlowPrefixMap on
ConfigCommands. The caller now does a direct map lookup against the
parsed prefix → branchType map instead of iterating the raw config
output and suffix-matching. This is preparation for adding git-flow-next
support, which needs to merge a second config schema into the same map.
One incidental change: a branch name without a slash now returns
NotAGitFlowBranch immediately, rather than falling through the
line loop with an empty suffix. Previously a configured
gitflow.prefix.X whose value happened to equal the entire branch
name could match — never a useful outcome.
