[FIX] account: backport of 12a39db8

[FIX] account: Preserve analytic account on tax lines which are on same general account as invoice line

After careful analysis, I'm now convinced it is a good thing to preserve
the analytic account on taxes line which have the same general account
as the invoice line.

This is the best default case and will save time for users,
while leaving the flexibility to adapt the analytic account on
taxes manually.