Sync with 2.18.2

* maint-2.18: (33 commits)
  Git 2.18.2
  Git 2.17.3
  Git 2.16.6
  test-drop-caches: use `has_dos_drive_prefix()`
  Git 2.15.4
  Git 2.14.6
  mingw: handle `subst`-ed "DOS drives"
  mingw: refuse to access paths with trailing spaces or periods
  mingw: refuse to access paths with illegal characters
  unpack-trees: let merged_entry() pass through do_add_entry()'s errors
  quote-stress-test: offer to test quoting arguments for MSYS2 sh
  t6130/t9350: prepare for stringent Win32 path validation
  quote-stress-test: allow skipping some trials
  quote-stress-test: accept arguments to test via the command-line
  tests: add a helper to stress test argument quoting
  mingw: fix quoting of arguments
  Disallow dubiously-nested submodule git directories
  protect_ntfs: turn on NTFS protection by default
  path: also guard `.gitmodules` against NTFS Alternate Data Streams
  is_ntfs_dotgit(): speed it up
  ...

git-submodule.sh

@@ -34,6 +34,7 @@ reference=
 cached=
 recursive=
 init=
+require_init=
 files=
 remote=
 nofetch=
@@ -447,6 +448,10 @@ cmd_update()
 		-i|--init)
 			init=1
 			;;
+		--require-init)
+			init=1
+			require_init=1
+			;;
 		--remote)
 			remote=1
 			;;
@@ -529,6 +534,7 @@ cmd_update()
 		${reference:+"$reference"} \
 		${dissociate:+"--dissociate"} \
 		${depth:+--depth "$depth"} \
+		${require_init:+--require-init} \
 		$recommend_shallow \
 		$jobs \
 		"$@" || echo "#unmatched" $?