Switch from manually running go generate to using dh-golang to run it

This fixes #2013.  For some reason I could only reproduce the issue from
within the docker builder, not when running outside of docker.
Regardless, this appears to fix the problem and is a generally cleaner
solution anyway that will continue to work if any other generate
commands are added to git-lfs.

debian/rules

@@ -18,6 +18,7 @@ BUILD_DIR := obj-$(DEB_HOST_GNU_TYPE)
export DH_GOPKG := github.com/git-lfs/git-lfs
# DH_GOLANG_EXCLUDES typically incorporates vendor exclusions
export DH_GOLANG_EXCLUDES := test github.com/olekukonko/ts/* github.com/xeipuuv/* github.com/spf13/cobra/* github.com/kr/* github.com/pkg/errors github.com/alexbrainman/sspi/*
export DH_GOLANG_GO_GENERATE := 1
export PATH := $(CURDIR)/$(BUILD_DIR)/bin:$(PATH)

# by-default, dh_golang only copies *.go and other source - this upsets a bunch of vendor test routines
@@ -32,7 +33,6 @@ override_dh_clean:
	dh_clean

override_dh_auto_build:
	cd ${BUILD_DIR}/src/github.com/git-lfs/git-lfs && go generate ./commands
	dh_auto_build
	#dh_golang doesn't do anything here in deb 8, and it's needed in both
	if [ "$(DEB_HOST_GNU_TYPE)" != "$(DEB_BUILD_GNU_TYPE)" ]; then\