Function get_deferred_commits_path

pub fn get_deferred_commits_path(repo: &Repo) -> PathBuf

Get the path to the file which stores the list of “deferred commits”.

During a rebase, we make new commits, but if we abort the rebase, we don’t want those new commits to persist in the smartlog, etc. To address this, we instead queue up the list of created commits and only confirm them once the rebase has completed.

Note that this has the effect that if the user manually creates a commit during a rebase, and then aborts the rebase, the commit will not be available in the event log anywhere. This is probably acceptable.