all: migrate go-git to new package
go-git now lives at github.com/go-git/go-git. v5 exists to allow the
migration to the new location and is API compatible with v4 [1].
[1]: https://github.com/go-git/go-git/releases/tag/v5.0.0
Change-Id: If68ba927e38ae8ef81447f995e449994928f0cfa