update portfolio value cache when adding stocks in the past

[wsteitz]

our portfolio-value-cache might have wrong values if a positon is added at a earlier date.

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Imported from Launchpad using lp2gh.

• date created: 2011-08-04T11:51:53Z
• owner: wolfer7
• the launchpad url was https://bugs.launchpad.net/bugs/820889