rebase --upstack hangs, performing repeated "git diff -name-only" operations in the background

[jdm-twtr]

I created two branches in a stack, then added an intentional conflict in the first branch. I performed gh stack rebase --upstack and it fails to provide any output other than:

gs rebase --upstack
✓ Fetched origin
Stack detected: (main) <- test/s1 <- test/s2
Rebasing branches in order, starting from test/s1 to test/s2

pstree -s rebase shows this over a handful of runs:

\--- 15004 git diff --name-only --diff-filter=U
...
\--- 15699 git diff --name-only --diff-filter=U
...
\--- 16264 git diff --name-only --diff-filter=U

[jdm-twtr]

Alright, this finished after SIX MINUTES:

⚠ Rebasing test/s1 onto main — conflict

But six minutes is not acceptable.

[jdm-twtr]

Ooof, thousands of files in untracked now. Something went really wrong here.

[jdm-twtr]

Things seemed to work fine when doing gs rebase --downstack then gs rebase --upstack. Which doesn't really make sense because on s1, it seems that --upstack would include both the current branch and everything upstack based on the help text:

--upstack         Only rebase branches from current branch to top