buildman: Allow selection of the number of commits to build

It is useful to be able to build only some of the commits in a branch. Add
support for the -c option to allow this. It was previously parsed by
buildman but not implemented.

Suggested-by: York Sun <yorksun@freescale.com>
Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: York Sun <yorksun@freescale.com>