fix: update apidiff script to install tool and set up remote repository

Author: yongruilin

Diff for: .gitignore

@@ -1 +1,2 @@
 .idea
+_output/

Diff for: hack/apidiff.sh

@@ -53,16 +53,33 @@ if [ "$#" -ge 1 ]; then
     TARGET_DIR="$1"
 fi
 
+# Create a temporary local GOBIN under _output/
+LOCAL_GOBIN="$(pwd)/_output/bin"
+mkdir -p "${LOCAL_GOBIN}"
+
 # Check for apidiff tool, install it if not found
 if ! command -v "${API_DIFF_TOOL}" &> /dev/null; then
+    GOBIN=${LOCAL_GOBIN}
     echo "Installing apidiff into ${GOBIN}."
-    go install golang.org/x/exp/cmd/apidiff@latest
+    GOBIN=${LOCAL_GOBIN} go install golang.org/x/exp/cmd/apidiff@latest
+    # Add GOBIN to PATH
+    export PATH=$PATH:${GOBIN}
 fi
 
+# Ensure the remote is set
+if ! git remote get-url origin &> /dev/null; then
+    echo "Error: 'origin' remote is not set. Please add the remote using:"
+    echo "git remote add origin https://github.com/kubernetes-sigs/structured-merge-diff.git"
+    exit 1
+fi
+
+# Fetch the latest changes from the remote
+git fetch origin
+
 # Fetch common base if -r is not set
 if [ -z "${REFERENCE_REVISION}" ]; then
-    echo "Determining common base with origin/master..."
-    REFERENCE_REVISION=$(git merge-base origin/master HEAD)
+    echo "Determining common base with upstream/master..."
+    REFERENCE_REVISION=$(git merge-base upstream/master HEAD)
 fi
 
 # Step 1: Create a temporary directory for worktrees