Sync with 2.21.1

* maint-2.21: (42 commits)
  Git 2.21.1
  mingw: sh arguments need quoting in more circumstances
  mingw: fix quoting of empty arguments for `sh`
  mingw: use MSYS2 quoting even when spawning shell scripts
  mingw: detect when MSYS2's sh is to be spawned more robustly
  t7415: drop v2.20.x-specific work-around
  Git 2.20.2
  t7415: adjust test for dubiously-nested submodule gitdirs for v2.20.x
  Git 2.19.3
  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
  ...

git-submodule.sh

@@ -36,6 +36,7 @@ reference=
 cached=
 recursive=
 init=
+require_init=
 files=
 remote=
 nofetch=
@@ -466,6 +467,10 @@ cmd_update()
 		-i|--init)
 			init=1
 			;;
+		--require-init)
+			init=1
+			require_init=1
+			;;
 		--remote)
 			remote=1
 			;;
@@ -548,6 +553,7 @@ cmd_update()
 		${reference:+"$reference"} \
 		${dissociate:+"--dissociate"} \
 		${depth:+--depth "$depth"} \
+		${require_init:+--require-init} \
 		$recommend_shallow \
 		$jobs \
 		-- \