repo download: add --ff-only option

Allows to ff-only a gerrit patch
This patch is necessary to automatically ensure that the patch will
be correctly submitted on ff-only gerrit projects

You can now use:
repo download (--ff-only|-f) project changeid/patchnumber

This is useful to automate verification of fast forward status of a patch
in the context of build automation, and commit gating (e.g. buildbot)

Change-Id: I403a667557a105411a633e62c8eec23d93724b43
Signed-off-by: Erwan Mahe <erwan.mahe@intel.com>
Signed-off-by: Pierre Tardy <pierre.tardy@intel.com>
2 files changed