Implement Clone-as-Submodule feature

[yasuo-ozu]

Background

There are some needs to manage your ghq root as git repository, for syncing your local stubs, etc.

Changes

This PR changes the default behavior of ghq get like:

• Check if the target directory is inside git worktree of another git repository.
• If so, use git submodule add instead.

[Songmu]

Thanks for the pull request.
It's an exciting idea to git manage the whole ghq root, but I don't hear much else about the need for it, and honestly, I'm not so positive about including this feature.
I'll stay open to it for a while, but if no one appears who agrees with this feature, I'll close it later.

[yasuo-ozu]

Thanks for reply. I am considering following situations.

• Using multiple devices (for example, note pc and desktop pc)
• I want to manage local repos, which I am working now.
• ~/ghq directory is a git repo (origin is set to my github.com private repo)
  (local repos managed in ~/ghq/ may be anything, like my public repo, my private repo or other's public repo (just looking at).)
• I can sync the working state between devices, using push/pull mechanism and working branch.
• Here, 'working state' is repos which I am working on, and working branches which I'm creating temporary commits in.

• Using 3rd-party automatic file sync (like Dropbox) may break local repos.
• Using tools like rsync is troublesome, and lack of collision fixing.

[yasuo-ozu]

I think working directly on ~/ghq/ is enough considerable, but someone may have different opinions.
If there is any other way (or reason why this example is unsound), please let me know.