Merge lineage not represented in stack view

[ErichDonGubler]

Given this topology shown by git-glimpse --format='%D' locals:

*   HEAD -> a-and-b
|\
| * a
| * main
* | b
|/
| * d
|/
*

…one would expect git-glimpse stack to output something like this:

*   HEAD -> a-and-b
|\
| * a
| * main
* | b
|/
*

…but the current output of git-glimpse stack only shows a single merge parent:

* HEAD -> a-and-b
* a
* main