[PATCH] Add update target to ninja

[Justin Bogner]

Gautier DI FOLCO <gautier.difolco at gmail.com> writes:
> I just added an update command to ninja, now we can do $ ninja update,
> as we can do $ make update.

Given that we recently removed 'make update' from the docs in r202430
and the inherent limitations with having this be part of the build
system (out of tree builds, git mirrors...), I'm not convinced we want
to do this.