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/services/repositories_test.go

@@ -44,8 +44,9 @@ func TestRepositoryForkSendsOrganizationOnlyWhenSelected(t *testing.T) {
 			w.WriteHeader(http.StatusAccepted)
 			_, _ = w.Write([]byte(`{"id":2,"full_name":"bob/project","fork":true,"parent":{"id":1,"full_name":"alice/project"}}`))
 		}))
-		item, err := (Repositories{Client: client}).Fork(context.Background(), "alice/project", org)
+		item, existed, err := (Repositories{Client: client}).Fork(context.Background(), "alice/project", ForkOptions{Organization: org})
 		require.NoError(t, err)
+		require.False(t, existed)
 		require.Equal(t, "alice/project", item.Parent.FullName)
 	}
 }