dir: new flag to remove_dir_recurse() to spare the original_cwd

remove_dir_recurse(), and its non-static wrapper called
remove_dir_recursively(), both take flags for modifying its behavior.
As with the previous commits, we would generally like to protect
the original_cwd, but we want to forced user commands (e.g. 'git rm -rf
...') or other special cases to remove it.  Add a flag for this purpose.
After reading through every caller of remove_dir_recursively() in the
current codebase, there was only one that should be adjusted and that
one only in a very unusual circumstance.  Add a pair of new testcases to
highlight that very specific case involving submodules && --git-dir &&
--work-tree.

Acked-by: Derrick Stolee <stolee@gmail.com>
Acked-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

dir.h

@@ -495,6 +495,9 @@ int get_sparse_checkout_patterns(struct pattern_list *pl);
 /* Remove the contents of path, but leave path itself. */
 #define REMOVE_DIR_KEEP_TOPLEVEL 04
 
+/* Remove the_original_cwd too */
+#define REMOVE_DIR_PURGE_ORIGINAL_CWD 0x08
+
 /*
  * Remove path and its contents, recursively. flags is a combination
  * of the above REMOVE_DIR_* constants. Return 0 on success.