Include package names in commit summary from cros_portage_upgrade.
Include package names in commit summary from cros_portage_upgrade.

When the number of packages is small (< 6) include the names of each
package in the commit summary (as well as the message body).

BUG=chromium-os:21105
TEST=Prepare commits with 1, 2, and 6 packages involved.
Inspect commit message summary.

Change-Id: I22571506b74adf50823a3f8fdf4325a0fb27c58f
Reviewed-on: http://gerrit.chromium.org/gerrit/8667
Reviewed-by: Matt Tennant <46e612415b446ccde19638e4035892001c237942@chromium.org>
Tested-by: Matt Tennant <46e612415b446ccde19638e4035892001c237942@chromium.org>
Commit-Ready: Matt Tennant <46e612415b446ccde19638e4035892001c237942@chromium.org>

zhang0137/chromite,coreos/chromite,bpsinc-native/src_third_party_chromite,bpsinc-native/src_third_party_chromite,chadversary/chromiumos.chromite,bpsinc-native/src_third_party_chromite,chadversary/chromiumos.chromite,zhang0137/chromite,coreos/chromite,zhang0137/chromite,coreos/chromite