Merge branch 'cp/submodule-custom-update'

In addition to the choice from "rebase, merge, or checkout-detach",
allow a custom command to be used in "submodule update" to update
the working tree of submodules.

* cp/submodule-custom-update:
  submodule update: allow custom command to update submodule working tree

diff --cc Documentation/git-submodule.txt
Simple merge