Merge branch 'hv/submodule-update-nuke-submodules' into maint

"git add" allows adding a regular file to the path where a submodule
used to exist, but "git update-index" did not allow an equivalent
operation to Porcelain writers.

* hv/submodule-update-nuke-submodules:
  update-index: allow overwriting existing submodule index entries

diff --cc builtin/update-index.c
Simple merge