remove_dir_recursively(): Add flag for skipping removal of toplevel dir

remove_dir_recursively(): Add flag for skipping removal of toplevel dir

Add the REMOVE_DIR_KEEP_TOPLEVEL flag to remove_dir_recursively() for
deleting everything inside the given directory, but _not_ the given
directory itself.

Note that this does not pass the REMOVE_DIR_KEEP_NESTED_GIT flag, if set,
to the recursive invocations of remove_dir_recursively().  It is likely to
be a a bug that has been present since REMOVE_DIR_KEEP_NESTED_GIT was
introduced (a0f4afb), but this commit keeps the same behaviour for now.

Signed-off-by: Johan Herland <johan@herland.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>