use matching sytest branch, or develop

jenkins/test-monolith.sh

@@ -13,12 +13,21 @@ rm -f sytest/server-*/*.log sytest/results.tap
 ./jenkins/prepare-dendrite.sh
 
 if [ ! -d "sytest" ]; then
-    git clone https://github.com/matrix-org/sytest.git --depth 1 --branch dendrite
+    git clone https://github.com/matrix-org/sytest.git --depth 1 --branch master
-else
-    git -C sytest fetch --depth 1 origin dendrite
-    git -C sytest reset --hard FETCH_HEAD
 fi
 
+# Jenkins may have supplied us with the name of the branch in the
+# environment. Otherwise we will have to guess based on the current
+# commit.
+: ${GIT_BRANCH:="origin/$(git rev-parse --abbrev-ref HEAD)"}
+
+git -C sytest fetch --depth 1 origin "${GIT_BRANCH}" || {
+    echo >&2 "No ref ${GIT_BRANCH} found, falling back to develop"
+    git -C sytest fetch --depth 1 origin develop
+}
+
+git -C sytest reset --hard FETCH_HEAD
+
 ./sytest/jenkins/prep_sytest_for_postgres.sh
 
 ./sytest/jenkins/install_and_run.sh \