add/discover: support numeric depth: N (follow-up to #550)

[tony]

Summary

Follow-up to #550 (shallow clone detection). That work records shallowness as a
boolean shallow: true, because the libvcs sync path can only express
shallow vs. full (git clone --depth 1). Once libvcs can honor an arbitrary
clone depth, add/discover should be able to persist a numeric depth: N and
expose a --depth N flag.

Background

• add/discover: detect shallow clones and persist depth to config #550 adds boolean shallow: true, mapped to git_shallow on sync.
• libvcs's GitSync.obtain() hardcodes depth=1 if self.git_shallow else None,
  so a numeric depth cannot be honored on sync today.
• Tracking issue on the backend: GitSync: honor arbitrary clone depth on sync, not just --depth 1 libvcs#531.

Proposal (once libvcs supports it)

1. Add a depth: int per-repository config key alongside shallow.
2. Detect the existing depth on add/discover (read .git/shallow /
   git rev-list --count) and record it.
3. Add --depth N to add/discover; persist depth: N and plumb it through
   to GitSync(depth=N) on sync.
4. Define precedence between shallow: true and depth: N (e.g. depth wins;
   shallow: true is sugar for depth: 1).

Why

shallow: true covers the common "I don't need history" case, but workspaces
that want a small window of history (e.g. --depth 50) currently can't express
it. This depends on the libvcs change above.