Possibly disable gc during actions

[joshmoore]

Since some of the commands run by scc are quite complicated, having git gc run in the middle can be disconcerting. We could temporarily disable it in .git/config as long as it gets re-enabled after each execution. Perhaps there's an environment variable that we could set instead.

Suggested by: @jburel

[sbesson]

Automatic git gc can be disabled with the following command:

git config gc.auto 0

Ideally, this PR should combine this command with the self.cd(self.path) operations within a decorator of the GitRepository class and restore the default value of the gc.auto flag (which should be saved).