Skip fetch when no remote is configured in `start` command

[alexrinass]

When gitflow.<type>.start.fetch is enabled on a local-only repo (no remote configured), the start command calls git.Fetch() without checking if the remote exists, producing a noisy warning ("does not appear to be a git repository"). Same class of bug as #69.

Expected Behavior

Fetch should be silently skipped when no remote is configured, matching the behavior introduced in #74 for finish. Add a git.RemoteExists(remoteName) guard before the fetch call at cmd/start.go:122-127.