LU-1053 build: add test for commit-msg with diff

LU-1053 build: add test for commit-msg with diff

Add a test case for the build/commit-msg script with "diff" at the
start of an intermediate line, which should not be ignored.  This
needs to be distinguished from the start of "git commit -av" output
which has a full "git diff" of the output inline with the commit.

Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: Icf2399568437de7f9e23419386bf3b1b99ebbfd1
Reviewed-on: http://review.whamcloud.com/3363
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Bruce Korb <bruce_korb@xyratex.com>
Reviewed-by: Yu Jian <yujian@whamcloud.com>