Use depth option when doing fetches
Use depth option when doing fetches

Previously the `depth` flag was only used for the initial clone-- so
although you might start with a depth=10 as more commits show up you are
always >10. With this diff we enforce that depth on each fetch, this way
old-commits can get GCd off to reduce the size of the local checkout
required.

kubernetes/git-sync,kubernetes/git-sync