Without base_commit, export diffs against the current guest HEAD.
If the worker ran git commit inside the VM, HEAD advanced and the
diff came back empty — committed work was silently lost.
With base_commit set to the head_commit from workspace.prepare,
the diff uses that fixed point instead. After git add -A the index
holds the full working state, so git diff --cached <base_commit>
captures everything: committed deltas (HEAD moved past base) and
any uncommitted changes on top, in one patch, applied with the
same git apply flow.
- WorkspaceExportParams gains base_commit
- WorkspaceExportResult echoes back the ref actually used
- CLI gains --base-commit flag
- Tests assert scripts use the caller-supplied ref and that
omitting it falls back to HEAD