sigh: add an option to 'watch' to specify the watched dir (#3120)
sigh: add an option to 'watch' to specify the watched dir (#3120)

This is useful on more restricted systems (like laptops) that can hit
file limits trying to watch the entire arcs repo.
PolymerLabs/arcs,PolymerLabs/arcs,PolymerLabs/arcs,PolymerLabs/arcs,PolymerLabs/arcs,PolymerLabs/arcs,PolymerLabs/arcs,PolymerLabs/arcs