Support for git fetch depth

[sirnewton01]

Go get will sometimes fetch an individual commit so that it can get the snapshot of the files from it. In this case there's no need to fetch the entire git history. Dgit currently fetches all of the commits leading up to that commit incurring the costs of that in terms of disk space and time.

It would be a tremendous performance optimization if dgit would support the depth parameter and allow the shallow fetch of just that commit.

Since the commits would no longer be guaranteed to have all of their preceding commits there would need to be some additional checking for "grafted" commits in git log. Note that when there's a grafted commit the git log should show the keyword "grafted" in the list of ref names in brackets ().

[driusan]

If we support this, we probably also need to support --deepen. Since it's mostly the remote that's responsible for calculating the objects and we don't use our own git-upload-pack implementation, I think most of the work is in reading the pack protocol documentation and figuring out where to insert the right line as part of the negotiation and determining whether it depends on any capabilities being advertised by the remote, or if it's always available