bug#20137: number of generation doesn't always rise monotonically

[Tomáš Čech]

Generation number is not always the maximum of all generation numbers
and so generation number is not always monotonic.

Steps to reproduce:

Lets start with on generation N.

1] install some package (you'll have N and N+1)
2] install some other package (you'll have N, N+1 and N+2)
3] delete generation N+1 (you'll have N and N+2)
4] switch to generation N
5] install some package - you'll get generation N+1 again
  (you'll have N, N+1 and N+2 again)

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