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/git/sync.go

@@ -61,7 +61,7 @@ func SyncBranch(ctx context.Context, runner Runner, opts SyncOptions) (SyncResul
 		return SyncResult{}, err
 	}
 	fetch := append([]string{}, opts.CredentialArgs...)
-	fetch = append(fetch, "fetch", "--no-tags", "--", opts.Remote, ref)
+	fetch = append(fetch, "fetch", "--quiet", "--no-tags", "--", opts.Remote, ref)
 	if _, err := run(fetch...); err != nil {
 		return SyncResult{}, err
 	}