mungedocs should not assume upstream remote

The standard Git convention is "origin", not "upstream", so make it
flexible in the scripts for those who are set up differently.

hack/lib/init.sh

@@ -40,6 +40,8 @@ source "${KUBE_ROOT}/hack/lib/version.sh"
 source "${KUBE_ROOT}/hack/lib/golang.sh"
 source "${KUBE_ROOT}/hack/lib/etcd.sh"
 
+KUBE_GIT_UPSTREAM="${KUBE_GIT_UPSTREAM:-upstream}"
+
 KUBE_OUTPUT_HOSTBIN="${KUBE_OUTPUT_BINPATH}/$(kube::util::host_platform)"
 
 # emulates "readlink -f" which is not available on BSD (OS X).
@@ -48,7 +50,7 @@ function readlinkdashf {
   # Follow links until there are no more links to follow.
   while readlink "$path"; do
     path="$(readlink $path)"
-  done 
+  done
   # Convert to canonical path.
   path=$(cd "$(dirname "${path}")" && pwd -P)
   echo "$path"