With the old system `checkpatch.pl` gets one sole input that consists of
the commit message and commit diff of each commit between BASE_COMMIT
and HEAD. It also filters out changes in some files, which makes `git
format-patch` completely ignore that commit, even the commit message.

With the new system the commit message and commit diff are analyzed
separately. This means that, even if all the files modified by a commit
are filtered out, the commit message will still be analyzed.

Also, all commits are analyzed individually. This way it's easier to
know which commit caused the problem, and there are no warnings about
repeated "Signed-off-by" lines.

Change-Id: Ic676a0b76801bb2607141a8d73dc3a942dc01c0e
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>