Remove git directory by default on checkout

[jaudiger]

This PR update the default behaviour on Git checkout when using gitCheckout from git recipe or Brioche.gitCheckout. As of today, the .git folder is always present after a checkout, but most of the time, we don't need this folder to be present for the recipe's source. Some recipes do need it to retrieve their version, but that's not the majority.

One of the consequence to have .git folder present in the source is it introduces some variances in the filesystem. On multiple git checkouts from the same remote, the output's hash will be each time different. .git folder is not designed to be fixed, it embeds some metadata such as timestamps, which prevent its reproducibility even on a shallow clone.

One direct impact of this source hash variance is each time we update git recipe or one of its dependencies, Brioche rebuilds all the recipes relying on gitCheckout which can take multiple hours. With this PR, we will remove one source of variance in the recipe's source, and hopefully reduce the build time of the CI.

A new option has been introduced keepGitDir to keep the .git folder when the recipes need it.

[jaudiger]

@kylewlacy I didn't broadly set keepGitDir. I would prefer to try putting this PR in the merge queue, and see the recipes that really need it after each CI failures.

[kylewlacy]

LGTM