Don't update dependencies of repos.

When updating repo A.com/b/c/..., only update the given repo and not all of its dependencies. This fixes #39.

It's still possible to update existing dependencies individually. But it won't fetch new repos, which may or may not be desired. User can always go get new repos when needed.

Previously, the behavior erred on side of "possibly updating/fetching too many repos at once" while the new behavior errs on side of "possibly not fetching enough new repos". But I think it's a safer side to be on for optimal user experience. It's also more predictable behavior.

As a very noticeable benefit, doing repo updates is now much, much faster, since it's simply doing a git pull --ff-only (or equivalent for other vcs) for one repo, instead of go get -u -d repo/... which did that, but also parsed all code and its dependencies, and updated their repos as well.

A future enhancement may be to detect dependencies of repos and display them in the visual presentation, allowing user to check/uncheck updating dependencies as they wish (or simply display which dependencies there are in need of updating).

shurcooL / Go-Package-Store

Don't update dependencies of repos. #40