jenkins: fix: recent change broke jenkins build

Recent commit 851802b927ccfcb061a2774fdc5de12141426b5c introduces a build
branch, which works fine, but only on the first run. A second run in the same
repos can't delete the branch git is currently on.

I've had enough of git being impossibly difficult in UI to simply checkout a
hash or a branch now. Completely wipe out the git repository and clone a fresh
one every time.

Unfortunately, 'git clone -b' doesn't allow passing a commit hash, so we still
need to do a clone-then-checkout dance. At least now we know it will work the
same on every run.

Change-Id: I6aca4c53a796312248a189b815dfc1198a173ed9
diff --git a/contrib/jenkins-build-common.sh b/contrib/jenkins-build-common.sh
index 779f965..895cd63 100644
--- a/contrib/jenkins-build-common.sh
+++ b/contrib/jenkins-build-common.sh
@@ -73,18 +73,17 @@
   fi
 
   cd "$base"
-  if [ ! -d "$repo" ]; then
-    git clone "$git_url/$repo" "$repo"
-  fi
-  cd "$repo"
-  git fetch origin
+  rm -rf "$repo"
+  git clone "$git_url/$repo" "$repo"
 
-  # Figure out whether we need to prepend origin/ to find branches in upstream
+  cd "$repo"
+
+  # Figure out whether we need to prepend origin/ to find branches in upstream.
+  # Doing this allows using git hashes instead of a branch name.
   if git rev-parse "origin/$branch"; then
     branch="origin/$branch"
   fi
 
-  git branch -D build_branch || true
   git checkout -b build_branch "$branch"
   rm -rf *
   git reset --hard "$branch"