LU-2961 build: Fix commit-msg to love new files

It is annoying to receive errors like this when the changes being
committed include new files:

  $ git commit -asv --amend
  line 65: diff --git a/lustre/include/lustre_errno.h
  b/lustre/include/lustre_errno.h
  new file mode 100644
  error: commit message invalid signoff section line
  line 66: index 0000000..89b8d1b
  error: commit message invalid signoff section line
  line 67: --- /dev/null
  error: commit message invalid signoff section line
  ...

This patch adds the "new file mode" pattern to commit-msg.  Hopefully,
this is enough.

Signed-off-by: Li Wei <wei.g.li@intel.com>
Change-Id: I39c985d51afb04184322a8fc725bcef3938bf419
Reviewed-on: http://review.whamcloud.com/5712


Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Hudson
Reviewed-by: Bruce Korb <bruce_korb@xyratex.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>