Merge branch 'dt/submodule-rm-with-stale-cache' into maint

Running "git rm" on a submodule failed unnecessarily when
.gitmodules is only cache-dirty, which has been corrected.

* dt/submodule-rm-with-stale-cache:
  git rm submodule: succeed if .gitmodules index stat info is zero

diff --cc submodule.c
Simple merge