Add -m/--modified to show files that have been modified wrt. the index.

M was already taken so the tag for modifified files is C (changed).

$ git-ls-files -m -t
C Documentation/git-ls-files.txt
C ls-files.c

Signed-off-by: Brian Gerst <bgerst@didntduck.org>

---

  Documentation/git-ls-files.txt |    4 ++++
  ls-files.c                     |   20 +++++++++++++++-----
  2 files changed, 19 insertions(+), 5 deletions(-)