Gauntlet update to reduce hash changes

[a-frantz]

The current issue: Everytime a gauntlet or arena repo pushes a new commit, we can get hundreds of lines show up in the git diff but none of them we care about. This leads to a situation where diagnostic changes we care about may get lost in the noise. It also means PRs end up with a bunch of git changes that are unrelated to the PR.

The fix: (originally a comment left on #194 that I have now expanded on)
I envision there being two types of changes to Gauntlet.toml/Arena.toml and they won't overlap in the same PR:

1. diagnostic changes (without hash changes)
   • these changes happen in our "typical" PRs (for lack of a better word)
   • New lint diagnostics, changes in diagnostic language, etc.
   • These are the changes we really care about
2. hash changes (which may or may not include diagnostic changes)
   • these will happen in dedicated PRs that run maybe once a week?
     • Could be automated, could be manual
   • any diagnostic changes happening here, we will know is because of a change "upstream" and not because of a regression in our code

To facilitate this, gauntlet will need some changes. The "regular" and "arena" run modes will need to stop pulling in new changes. --refresh could be renamed --bless. Then we'd need a new flag (which is mutually exclusive with --refresh/bless) that goes and updates all the commit hashes (probably just call it --update?).