Perform a shallow clone in the git provider for faster startup

[jawher]

Running katafygio for a couple of months on rather active cluster
results in a huge git repo.

On startup, katafygio clone the git repository, which in our case was
very slow, exceeding the configured deadlines.

This PR propose to perform a shallow clone with a depth of 1: katafygio
doesn't need to full history to work, and the startup is much snappier.

[coveralls]

Coverage remained the same at 89.651% when pulling 6bfebe6 on jawher:do-shallow-clone into 7b0af9e on bpineau:master.

[obourgain]

👍 need this

[bpineau]

Makes sense, thanks a lot Jawher!
Need a new release+image right now or did you already built yours?
Speaking of ever growing repository, I wonder whether we should offer the option to batch commits (ie. group and commit changes every n seconds)? Worth noting: filtering irrelevant objects (pods, replicasets, events, endpoints, ...) lowers the churn.

[jawher]

Thanks for merging this so quickly 👍

Need a new release+image right now or did you already built yours?

For the time being we are using a custom built image with a patched version.
No pressure though, we'll switch back as soon as you release a new version.

Speaking of ever growing repository, I wonder whether we should offer the option to batch commits (ie. group and commit changes every n seconds)

Sure, could be handy 👍