internal/charmstore: Minor stats improvement.

[mhemmings]

This change means that in a case where -1 revision (all revisions) is passed to ArchiveDownloadCounts, we half the number of db requests made.

[jujugui]

Refer to this link for build results (access rights to CI server needed): http://ci-gce.jujugui.org:8080//job/charmstore/2260/