Sync with 2.16.6

* maint-2.16: (31 commits)
  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
  mingw: disallow backslash characters in tree objects' file names
  path: safeguard `.git` against NTFS Alternate Streams Accesses
  ...

environment.c

@@ -75,7 +75,7 @@ enum log_refs_config log_all_ref_updates = LOG_REFS_UNSET;
 int protect_hfs = PROTECT_HFS_DEFAULT;
 
 #ifndef PROTECT_NTFS_DEFAULT
-#define PROTECT_NTFS_DEFAULT 0
+#define PROTECT_NTFS_DEFAULT 1
 #endif
 int protect_ntfs = PROTECT_NTFS_DEFAULT;
 const char *core_fsmonitor;