Pull git-flow prefix parsing into a config-level helper

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.