jenkins-build-common.sh: cosmetic: clear repos a bit later

If we rm -rf * and then checkout a branch, the log prints the entire file tree
as deleted. Instead, rm just before the git reset --hard, which avoids the
extra output.

Change-Id: Ib5b28a82f05d941eae8f3a2f468ef1e9d67e6180
diff --git a/contrib/jenkins-build-common.sh b/contrib/jenkins-build-common.sh
index 63e0ba8..779f965 100644
--- a/contrib/jenkins-build-common.sh
+++ b/contrib/jenkins-build-common.sh
@@ -77,7 +77,6 @@
     git clone "$git_url/$repo" "$repo"
   fi
   cd "$repo"
-  rm -rf *
   git fetch origin
 
   # Figure out whether we need to prepend origin/ to find branches in upstream
@@ -87,6 +86,7 @@
 
   git branch -D build_branch || true
   git checkout -b build_branch "$branch"
+  rm -rf *
   git reset --hard "$branch"
 
   git rev-parse HEAD