diff_filespec: reorder dirty_submodule macro definitions

diff_filespec: reorder dirty_submodule macro definitions

diff_filespec has a 2-bit "dirty_submodule" field and
defines two flags as macros. Originally these were right
next to each other, but a new field was accidentally added
in between in commit 4682d85. This patch puts the field and
its flags back together.

Using an enum like:

  enum {
	  DIRTY_SUBMODULE_UNTRACKED = 1,
	  DIRTY_SUBMODULE_MODIFIED = 2
  } dirty_submodule;

would be more obvious, but it bloats the structure. Limiting
the enum size like:

  } dirty_submodule : 2;

might work, but it is not portable.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>