ui: Checkout release branch if given

Change-Id: I0222b5d34fe6f6a95a52398bb86c931d7656a720

ui/release/builder_entrypoint.sh

diff --git a/ui/release/builder_entrypoint.sh b/ui/release/builder_entrypoint.sh
index 7352fa3..27340ea 100755
--- a/ui/release/builder_entrypoint.sh
+++ b/ui/release/builder_entrypoint.sh
@@ -31,18 +31,20 @@
 
 cd /workspace/
 
-# infra/ui.perfetto.dev/cloudbuild_release.yaml sets $1 to the branch
-# name.
-EXTRA_ARGS=""
-if [[ ! -z $1 ]]; then
-  EXTRA_ARGS="--branch_only=$1"
-fi
-
 ls -A1 | xargs rm -rf
 UPSTREAM="https://android.googlesource.com/platform/external/perfetto.git"
 git clone $UPSTREAM upstream
 
 cd upstream/
+
+# infra/ui.perfetto.dev/cloudbuild_release.yaml sets $1 to the branch
+# name.
+EXTRA_ARGS=""
+if [[ ! -z $1 ]]; then
+  git checkout $1
+  EXTRA_ARGS="--branch_only=$1"
+fi
+
 git rev-parse HEAD
 mkdir /workspace/tmp
 python3 -u "$CUR_DUR/build_all_channels.py" \