v4's show-progress flag should suppress "git checkout" as well

[ychin]

v4 recently introduced a "show-progress" flag that will suppress the "--progress" flag from Git fetch. However, this action will then perform a "git checkout" later with a hard-coded --progress flag. I think it makes sense to also remove it if show-progress: false is specified. I think it would be rare for someone to care about the progress in checkout but not fetch to begin with, so keeping this the same for both Git fetch and checkout will keep it simple.

Note: There's an existing bug where this flag currently doesn't do anything and always defaults to false (it should default to true) (#1453). This suggestion is assuming the bug got fixed.