Do not try to compute cap gains if cap gains not possible.

[Linas Vepstas]

Log Message:
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Do not try to compute cap gains if cap gains not possible.
Gains aren't possible if the lot commodity is the same as
the transaction currency.   I think that this should fix
http://bugzilla.gnome.org/show_bug.cgi?id=139677
Untested, let me know.

Modified Files:
--------------
    gnucash/src/engine:
        Scrub3.c