Fix #416 by allowing subtraction of zero-amount mismatched commodities

[bcc32]

This PR changes ledger-{add,subtract}-commodity to allow the second argument's
commodity string not to match the first argument's, as long as the second amount
is zero. Since subtracting a zero amount is a no-op anyway, this should be fine
in terms of correctness, and alleviates some spurious errors.

Fix #416

[purcell]

This feels like an indirect and unexpected fix to functions not directly related to ledger-reconcile. It's still a logical error to subtract/add mismatching commoditised values, whatever the numbers. I suspect there may be a better, more local, fix for #416.

[bcc32]

I did think of that, but I think this change is still "justified", in that:

1. These functions are only used by reconcile (in fact, ledger-add-commodity is not used at all)
2. ledger itself makes this simplification:

$ ledger eval '0 USD'
0

$ ledger eval '0 USD - 0'
0

$ ledger eval '0 USD - 0 GBP'
0

FWIW, the commodity representation in ledger-mode is not quite accurate anyway; ledger itself can represent multiple commodities in the same amount just fine, although the printing and reading syntax then becomes more complex:

$ ledger eval '1 USD - 1 GBP'
-1 GBP
1 USD

(so I suppose it is reasonable not to support it directly in ledger-mode)

If you think this change should be more localized to reconciliation, I can make that change too, although since it's ledger itself that's printing 0 instead of 0 USD (or whatever commodity), it feels a bit more hacky to me than just changing the subtraction code.

[purcell]

That makes sense. Happy for this change to be merged as-is, then.

[bcc32]

Will do, thanks for reviewing!