partial fix for http://bugzilla.gnome.org/show_bug.cgi?id=139677

[Linas Vepstas]

Log Message:
-----------
partial fix for http://bugzilla.gnome.org/show_bug.cgi?id=139677
requires that all splits in lot share a common transaction currency.

Modified Files:
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    gnucash/src/engine:
        policy.c