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

builtin/update-index.c

@@ -211,12 +211,6 @@ static int process_path(const char *path)
 	if (S_ISDIR(st.st_mode))
 		return process_directory(path, len, &st);
 
-	/*
-	 * Process a regular file
-	 */
-	if (ce && S_ISGITLINK(ce->ce_mode))
-		return error("%s is already a gitlink, not replacing", path);
-
 	return add_one_path(ce, path, len, &st);
 }