:tab-move end doesn't work when moving tabs from a subtree

[toofar]

Version info:

3bd272d on tree-tabs-integration

Does the bug happen if you start with --temp-basedir?:

With -s tab.tree_tabs true, yeah.

Description

Tabs moved from a subtree to the end of the tab bar don't quite end up at the end, they end up one tab short of the end. As spotted in #8102

I suspect it can be resolved by special casing index == "end" here. But should we be looking at the value of index in that part of the code when we are already doing so in the bit above? Also why doesn't it seem to be working for moving a tab that's already at the top of the tree to the end?

How to reproduce

Plop this in tests/end2end/features/treetabs.feature and run like python3 -m pytest tests/end2end/features/test_treetabs_bdd.py -k from_a_subtree_to_end:

    Scenario: Move a tab from a subtree to "end"
        When I open data/numbers/1.txt
        And I open data/numbers/2.txt in a new related tab
        And I open data/numbers/3.txt in a new tab
        And I open data/numbers/4.txt in a new tab
        And I run :tab-focus 2
        And I run :tab-move end
        Then the following tabs should be open:
          - data/numbers/1.txt
          - data/numbers/3.txt
          - data/numbers/4.txt
          - data/numbers/2.txt (active)

CC #8077 as it might end up getting fixed as part of that.