Make PR clones shallow and don't clone the branch first

[aidanhs]

Interim fix for travis-ci/travis-ci#7589. Read that issue for background on the approach here. This PR

• stops travis cloning a branch we don't care about before cloning the actual PR ref
• makes sure PR cloning obeys the depth configuration
• fixes an issue that made Display last GitHub status message if cloning fails #929 not work (I think?) because it specified assert on the command before the conditional

[aidanhs]

Rebased and seems to be failing on

rake aborted!
Could not find releases for tools/godep
/home/travis/build/travis-ci/travis-build/Rakefile:71:in `latest_release_for'
/home/travis/build/travis-ci/travis-build/Rakefile:148:in `block in <top (required)>'
Tasks: TOP => default => update_static_files => public/files/godep_darwin_amd64

which I don't think is my fault?

[aidanhs]

Forced a rebuild, seems fine now. Any chance of this getting a look at some point? We're seeing network failures on initial clone (over at the Rust repo) that would likely be solved (or at least mitigated) by this.

[meatballhat]

@aidanhs Hello and sorry about the very long delay ❤️ ! Assuming this work is still relevant and desired, are you willing to take a crack at resolving the conflicts?

[meatballhat]

I'm closing this out given the conflicts, and I'm not sure it's still applicable work. Please holler if you would like to see this PR reopened 💖 !