How do I keep the canonical current?

`gitto sync` fetches, fast-forwards it, and runs `.gitto-refresh` if the project carries one. `gitto list` prints how far behind the canonical is on its first line, so the drift is visible before it costs you anything.

gitto sync fetches, fast-forwards it, and runs .gitto-refresh if the project carries one. gitto list prints how far behind the canonical is on its first line, so the drift is visible before it costs you anything.