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