The diff tool relied on configuring which build system to use via BAZEL_BUILD_ENABLED. After the thin targets change, two build definitions are no longer permitted.

Since this tool is only intended to be run in CI manually when creating migration PRs, use git to stash the PR's changes locally to build the desired targets in SCons.

This relies on the fact that Evergreen stores the PR's changes as staged files, but also would work if Evergreen stored all of the PR's changes as a commit.