normal: make <c-o> always jump backward when snapshotting current selection

[lobre]

JumpList::backward used to sometimes consume <c-o> without moving: when the current selection had diverged from the jump at m_current, we first pushed the current selection, then subtracted count.

With count=1, this landed on the freshly pushed entry itself, so <c-o> behaved like "save checkpoint" instead of "jump backward".

This was especially visible after going back into the middle of history, moving, then pressing <c-o> again: the first <c-o> could be a no-op from the user point of view.

Fix by treating the implicit push as an extra internal step: if we push current before jumping, move back by count + 1. This keeps the "preserve current position for <c-i>" behavior, while making <c-o> consistently perform an actual backward jump.

Add regression test: test/normal/jump/backward-dirty-middle

Fixes #5066

[mawww]

Thanks, this looks like a nice cleanup of that function on top of fixing the bug.