Factor out some of the code shared between the sytest scripts (#974)

* Factor out some of the code shared between the different sytest jenkins scripts

* Exclude jenkins from the MANIFEST

* Fix dendron build

* Missing new line

* Poke jenkins

* Export the PORT_BASE and PORT_COUNT

* Poke jenkins

jenkins/clone.sh

@@ -0,0 +1,24 @@
+#! /bin/bash
+
+NAME=$1
+PROJECT=$2
+BASE=".$NAME-base"
+
+# update our clone
+if [ ! -d .$NAME-base ]; then
+  git clone $PROJECT $BASE --mirror
+else
+  (cd $BASE; git fetch -p)
+fi
+
+rm -rf $NAME
+git clone $BASE $NAME --shared
+
+: ${GIT_BRANCH:="origin/$(git rev-parse --abbrev-ref HEAD)"}
+cd $NAME
+# check out the relevant branch
+git checkout "${GIT_BRANCH}" || (
+    echo >&2 "No ref ${GIT_BRANCH} found, falling back to develop"
+    git checkout "origin/develop"
+)
+git clean -df .