Renaming away from git-apps

[stuartpb]

Discussion to revert #5

Per the developments in app-repo integration that are forming after plushu/plushu-git#9, this plugin is no longer responsible for most of "app lifecycle integration with Git" (most of which is now looking to be just handled as an effect of apps hooking into repos, with generic repo functionality handled by the main git plugin). This will specifically only handle the Git integration with apps as far as Git-specific interfaces go.

So maybe this should go back to being git-receive-app or git-update-apps or something like that.

[stuartpb]

In light of #9 I'm cool leaving this the way it is, at least for now.