Populate the DAG on git branchless init

[arxanas]

If you run git branchless init and then git smartlog in a big repository like https://github.com/mozilla/gecko-dev, it takes ~20 seconds to build the initial internal DAG. It would be best to do this during the init step, since that's the time you would expect any heavy-lifting operations to be carried out.

We just need to add a call to sync the DAG in the init function here:

git-branchless/src/commands/init.rs

Lines 550 to 556 in 45ace45

	/// Initialize `git-branchless` in the current repo.
	#[instrument]
	pub fn init(
	effects: &Effects,
	git_run_info: &GitRunInfo,
	main_branch_name: Option<&str>,
	) -> eyre::Result<()> {

The call is to Dag::open_and_sync, and looks like this:

git-branchless/src/commands/restack.rs

Lines 242 to 251 in 45ace45

	let references_snapshot = repo.get_references_snapshot()?;
	let event_replayer = EventReplayer::from_event_log_db(effects, &repo, &event_log_db)?;
	let event_cursor = event_replayer.make_default_cursor();
	let mut dag = Dag::open_and_sync(
	effects,
	&repo,
	&event_replayer,
	event_cursor,
	&references_snapshot,
	)?;

To test this:

1. Clone mozilla/gecko-dev locally.
2. Delete any existing DAG like this: rm -r /path/to/gecko-dev/.git/branchless/dag/
3. Run cargo run -- -C /path/to/gecko-dev init and confirm that it produces a progress meter like this:

Updating commit graph (5.1s)

and that it terminates after a few seconds.

It would also be cool if you could make the message say "Initializing commit graph" instead of "Updating commit graph" if this is the first time the DAG is being initialized, but it's not important. (That would be a more involved change.)