cros_portage_upgrade: Fix commit message generation when versions vary by arch
cros_portage_upgrade: Fix commit message generation when versions vary by arch

cros_portage_upgrade generated incorrect commit messages when upgrading
to different versions on different architectures; it would emit the same
architecture name multiple times in place of the architecture
corresponding to each version, like this:

Upgraded x/y to version 1 on x86 AND version 2 on x86

Fix the commit message generation to match up architectures and
versions; in the process, simplify it significantly, dropping the
distinction between the single-version and multi-version cases.  This
also improves the messages to group architectures by version.  The
result looks like this:

Upgraded x/y to version 1 on arm AND version 2 on amd64, x86

BUG=None
TEST=Use cros_portage_upgrade on a package with different versions
     available on different architectures, and check the resulting
     commit message.

Change-Id: I501769c5e085e0d9eefcd692da8520c45506b09d
Reviewed-on: https://gerrit.chromium.org/gerrit/32328
Tested-by: Josh Triplett <c028c213ed5efcf30c3f4fc7361dbde0c893c5b7@joshtriplett.org>
Reviewed-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>
Reviewed-by: Brian Harring <fdc2d0a2e2340b623587cee5ea8dff425d1eb1f3@chromium.org>
Commit-Ready: Josh Triplett <c028c213ed5efcf30c3f4fc7361dbde0c893c5b7@joshtriplett.org>

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