AccountingTransaction_guessGroupedLines : generate better grouping reference letters, using this scheme:
A, B, C ..., Z, AA, AB, ..., AZ, BA, BBB, ..., ZZ, AAA, AAB,
it was mistakenly using this one:
A, B, C ..., Z, AA, AB, ...AZ, AAA, AAB

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@20321 20353a03-c40f-0410-a6d1-a30d3c3de9de