feat(builder): Try shallow cloning buildpack repos

Try to shallow clone buildpack repositories before falling back to deep
cloning. This improves clone times significantly.

Author: johanneswuerbach

builder/image/slugbuilder/builder/install-buildpacks

@@ -8,10 +8,18 @@ download_buildpack() {
     buildpack_name=$(basename $buildpack_url)
     buildpack_commit="$2"
 
-    git clone $buildpack_url $BUILDPACK_INSTALL_PATH/$buildpack_name
-    pushd $BUILDPACK_INSTALL_PATH/$buildpack_name &>/dev/null
-        git checkout --quiet $buildpack_commit
-    popd &>/dev/null
+    set +e
+    git clone --branch $buildpack_commit --depth 1 $buildpack_url $BUILDPACK_INSTALL_PATH/$buildpack_name
+    SHALLOW_CLONED=$?
+    set -e
+    if [ $SHALLOW_CLONED -ne 0 ]; then
+        # if the shallow clone failed partway through, clean up and try a full clone
+        rm -rf $BUILDPACK_INSTALL_PATH/$buildpack_name
+        git clone $buildpack_url $BUILDPACK_INSTALL_PATH/$buildpack_name
+        pushd $BUILDPACK_INSTALL_PATH/$buildpack_name &>/dev/null
+            git checkout --quiet $buildpack_commit
+        popd &>/dev/null
+    fi
 
 }