[build] consider using git worktrees for bazel-diff

[aaylward]

Is it possible to make bazel-diff diff against origin/main directly instead of having to clone with some sufficient depth in CI?

⏺ Yes, you can modify bazel-diff to work without checking out revisions by using git's ability to show files at specific commits. Here are a few approaches:

⏺ Read(scripts/diff-build)
⎿  Read 15 lines (ctrl+r to expand)

⏺ Option 1: Use git worktree - Create temporary worktrees for each revision instead of checking out:

Instead of checkout, use worktrees

TEMP_WORKTREE="/tmp/bazel-diff-base-$$"
git worktree add "$TEMP_WORKTREE" "$BASE_REVISION"
"$BAZEL_DIFF_BIN" generate-hashes -w "$TEMP_WORKTREE" -b "$BAZEL_PATH" $EXTERNAL_REPOS "$starting_hashes_json"
git worktree remove "$TEMP_WORKTREE"

Option 2: Use git archive - Extract files at specific commits:

Extract base revision files

BASE_DIR="/tmp/bazel-diff-base-$$"
git archive "$BASE_REVISION" | tar -x -C "$BASE_DIR"
"$BAZEL_DIFF_BIN" generate-hashes -w "$BASE_DIR" -b "$BAZEL_PATH" "$starting_hashes_json"

Option 3: Fetch only what you need:

In CI, fetch just the base branch

git fetch origin main:origin/main --depth=1

The git worktree approach (Option 1) is cleanest since it doesn't require checking out different revisions in your main workspace, and bazel-diff can still run against proper git repositories rather than extracted archives.