fw update gets confused if it hits a disabled branch

[xmo-odoo]

Observed on odoo/odoo#271037:

• PR in 16.0, skipmerge'd, all PRs created
• one of the intermediate branches (18.2) disabled
• PR was updated

This caused the mergebot to create a second sequence of forward-ports right after the disabled branch.

Hypothesis is it didn't "see" the intermediate batch in the sequence, and looked for a batch in 18.3 child of the 18.0 batch, didn't find it, then created one. The batch id / branch names pretty much tell that story:

• the 18.0 batch has id 541996
• the 18.2 batch has id 541998, suffix 541996-fw
• the original 18.3 batch has id 541999, suffix 541999
• the new 18,3 batch has id 551301, suffix 541996-fw