keep using dbgsym when one diffs a .buildinfo or a .changes file

After #44 (closed), implemented in 5ef24a6a, diffing a deb doesn't take into account the relative -dbgsym packages that happen to be near them.

Whilst that may be what somebody wants in some occasion, I think that's a bad default. At least, when somebody diffs a .changes file (or a .buildinfo, or other similar "container") I think it would be best to keep using -dbgsym packages to improve the quality of the objdump output.

/cc @josch