Merge branch 'dm/submodule-update-i-shorthand'

The '-i' shorthand for the '--init' option, which was accepted by the
'git submodule update' command until it was broken in a modernization
of the option-parsing code, has been restored.

* dm/submodule-update-i-shorthand:
  submodule--helper: accept '-i' shorthand for update --init

diff --cc builtin/submodule--helper.c
Simple merge