The original parser that was used in #3825 brings in a lot of
dependencies that complicate packaging git-lfs. This replaces it with a
small parser I wrote with almost no dependencies. I've tested this as
extensively as i can and it seems to work correctly.
Vendor the `golang.org/x/net` dependency. This will be used in a
future commit to add HTTP/2 transport support.
Note that due to the way go modules work, the `golang.org/x/sys`
dependency was updated as well when running `make vendor`.