Add shallow clone option to git clone

[lyuha]

shallow clone can make faster than unshallow clone(fetching all histories of HEAD). Add options "--depth ", "--no-single-branch" to clone elm repos.

• "--depth " create shallow clone with a history truncated to of revisions.
• "--no-single-branch" fetch the histories near the tips of all branches.

[evancz]

Some people use these for development. The overall build time is so dominated by compilation that I don't think this is worth it.