Allow the pull-docs script to pull other than master

Author: bep

pull-docs.sh

@@ -1,5 +1,7 @@
 #!/bin/bash
 
+HUGO_DOCS_BRANCH="${HUGO_DOCS_BRANCH-master}"
+
 # We may extend this to also push changes in the other direction, but this is the most important step.
-git subtree pull --prefix=docs/ https://github.com/gohugoio/hugoDocs.git master --squash
+git subtree pull --prefix=docs/ https://github.com/gohugoio/hugoDocs.git ${HUGO_DOCS_BRANCH} --squash