Radically reduce first run pod setup bandwidth.

Given that Cocoapods doesn't need the history of the Specs repo we can radically improve the initial setup time by only checking out HEAD. This reduces the initial download from >200MB to roughly 3.5MB, a massive win for folks with slow interwebs.

git pull/git fetch+git rebase work as expected.
Update CHANGELOG.
Merge pull request #1803 from jverkoey/fastsetup

Radically reduce first run pod setup bandwidth.
[Command] Add `pod repo add --shallow` option. Default to false.
[Command] Setup shallow clones by default or use `--no-shallow`.
[Command] Setup full clone when using `--push`.
[CHANGELOG] Expand on shallow repo clones.