feat(repo): sync forks on the server, wire forks into clones, validate set-default

repo sync OWNER/REPO, run where no Git remote points to the repository,
now updates it on the server: a fork's branch fast-forwards to its
parent's through merge-upstream, and a pull mirror fetches its source.

repo fork reports an existing fork ("! bob/x already exists") instead of
a raw HTTP 409, refuses to fork a repository into its own owner, and
gains --fork-name, --clone (Git flags after --) and --remote, which adds
the fork as origin and renames the old origin to upstream.

repo clone passes Git flags after --, and takes --upstream-remote-name
and --no-upstream for forks. Clones report what they set up.

repo set-default checks that the repository exists before saving it,
prints the saved repository with --view, removes it with --unset, and
reports the change.

Signed-off-by: bircni <bircni@icloud.com>

internal/app/repository_create.go

@@ -140,7 +140,8 @@ func (a *application) repoCreateCommand() *cobra.Command {
 			return err
 		}
 		target.Repo = item.FullName
-		return a.cloneRepository(cmd, target, item, "", 0)
+		_, err = a.cloneRepository(cmd, target, item, cloneOptions{upstream: "upstream"})
+		return err
 	}}
 	flags := command.Flags()
 	flags.StringVar(&opts.Name, "name", "", "Repository name")