Sync with 2.23.1

* maint-2.23: (44 commits)
  Git 2.23.1
  Git 2.22.2
  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
  ...

git-compat-util.h

@@ -397,6 +397,10 @@ static inline int git_offset_1st_component(const char *path)
 #define offset_1st_component git_offset_1st_component
 #endif
 
+#ifndef is_valid_path
+#define is_valid_path(path) 1
+#endif
+
 #ifndef find_last_dir_sep
 static inline char *git_find_last_dir_sep(const char *path)
 {