fetch-all-upstreams: newly added, forked repos paged by name; update "through cache" from there

[salewski]

This issue depends on:

• issue cache: newly added, forked repos paged by name; update from there #65 ("cache: newly added, forked repos paged by name; update from there")

---

There is an idea in issue #65 to allow partial updating of the cache for the /user/repos "page collection" based on the (partial) name of the newly created or forked repo.

When cloning a GitHub repo, though, the tool the user is reaching for is ads-github-fetch-all-upstreams (with an invocation specifying the particular repo name(s) to clone), not ads-github-cache.

It would be nice to be able to do:

    $ ads-github-fetch-all-upstreams -vcu some-new-repo

and have it clone 'some-new-repo', even if it is not currently in the cache, and without having to pay the time penalty for updating the entire cache.

Such a feature would presumably rely on capabilities provided by issue #65.

The "Bonus points" section in issue #65 holds here, as well. The ideal solution would:

1. update the single relevant page in the cache (determined by (full) repo name)
2. immediately leverage the newly cached page to clone the specified repo
3. immediately return shell control to the user (to continue working; but...)
4. trigger or allow the background cache updating of "the rest of the pages" to proceed, notifying the user when complete.