Add --quiet switches to checkouts

[atesgoral]

Follow-up to #1219 and #1225. Missed a spot.

Output is garbled due to progress feedback of checkout:

Tophatted locally:

git -c advice.detachedHead=false checkout --quiet 05f5b2e2b72a80491d8ebe3e91a350a7ae09dc5e

[ajshepley]

This is something we might want to look at making customizable per-stack at some point (perhaps allowing stacks to override the whole git command itself might make sense?). But in any case, quiet by default makes sense.

[atesgoral]

@ajshepley I've been thinking about the same. Other consumers of shipit-engine might still make good use progress output. Might make sense to revise all the silencing we've added so far and add an option to toggle verbosity / progress.

[benlangfeld]

@mfilipe has an interest in contributing making these quiet flags configurable. In our use case we're currently running on a fork which removes them in order to avoid timing out log collection on long git clone operations.