Merge branch 'nd/fetch-depth-is-broken'

Merge branch 'nd/fetch-depth-is-broken'

"git fetch --depth" was broken in at least three ways.  The
resulting history was deeper than specified by one commit, it was
unclear how to wipe the shallowness of the repository with the
command, and documentation was misleading.

* nd/fetch-depth-is-broken:
  fetch: elaborate --depth action
  upload-pack: fix off-by-one depth calculation in shallow clone
  fetch: add --unshallow for turning shallow repo into complete one