add depth to git fetcher

[DennisDenuto]

LGTM

• Ran the tests locally
• Manual test works with a commit ref
• If a given depth doesn't contain the commit ref the error message is a bit cryptic but it is consistent with providing a commit ref that doesn't exist.

  Running 'vendir sync'...
  vendir.go:79: Failed to successfully execute 'vendir sync': Execution error: stdout: '' stderr: 'vendir: Error: Syncing directory 'vendor':
        Syncing directory 'deeper-sha' with git contents:
          Fetching git repository:
            Git [-c advice.detachedHead=false checkout 5d06e4025f13e1230be4df632b915a4233e0694d]: exit status 128 (stderr: fatal: reference is not a tree: 5d06e4025f13e1230be4df632b915a4233e0694d
      )

@cppforlife If you haven't already, I think a follow up PR to the carvel docs should be made.