Prevent integration tests changing git-lfs repo hooks dir

Previously running any tests inside the `git-lfs` repo would create
hooks which was undesirable (regardless of whether you use
GIT_LFS_TEST_DIR or not). Gets worse the more hooks we add.

test/testhelpers.sh

@@ -335,7 +335,7 @@ setup() {
   # Set up the initial git config and osx keychain if applicable
   HOME="$TESTHOME"
   mkdir "$HOME"
-  git lfs install
+  git lfs install --skip-repo
   git config --global credential.usehttppath true
   git config --global credential.helper lfstest
   git config --global user.name "Git LFS Tests"