Make sure objects don't get truncated if build is interrupted (#237)
Make sure objects don't get truncated if build is interrupted (#237)

When adding a file to the store, copy it to a temporary location first, then move it to the real one. Move is atomic, so should guarantee we don't end up with partial content.
quiltdata/quilt,quiltdata/quilt,quiltdata/quilt-compiler,quiltdata/quilt-compiler,quiltdata/quilt-compiler,quiltdata/quilt,quiltdata/quilt,quiltdata/quilt-compiler,quiltdata/quilt