$ git config set --global core.fsmonitor true

$ git clone --no-checkout git@github.com:tim-janik/jj-fzf.git
Cloning into 'jj-fzf'...
remote: Enumerating objects: 1525, done.
remote: Counting objects: 100% (276/276), done.
remote: Compressing objects: 100% (74/74), done.
remote: Total 1525 (delta 264), reused 207 (delta 202), pack-reused 1249 (from 2)
Receiving objects: 100% (1525/1525), 307.26 KiB | 2.77 MiB/s, done.
Resolving deltas: 100% (922/922), done.

# here is where we replicate the JJ fetch being done as part of the colocated clone.
# i've set GIT_TRACE to show where the hang happens relevant to the fsmonitor

$ LC_ALL=C GIT_TRACE=1 /usr/local/bin/git --git-dir /Users/dougstephen/git/jj-fzf/.git fetch --verbose --prune --no-write-fetch-head --progress -- origin '+refs/heads/*:refs/remotes/origin/*'
11:08:59.850406 git.c:476               trace: built-in: git fetch --verbose --prune --no-write-fetch-head --progress -- origin '+refs/heads/*:refs/remotes/origin/*'
11:08:59.851538 run-command.c:667       trace: run_command: git fsmonitor--daemon start
11:08:59.851564 run-command.c:759       trace: start_command: /usr/local/opt/git/libexec/git-core/git fsmonitor--daemon start
11:08:59.861526 git.c:476               trace: built-in: git fsmonitor--daemon start
11:08:59.862760 run-command.c:667       trace: run_command: git fsmonitor--daemon run --detach --ipc-threads=8
11:08:59.862785 run-command.c:759       trace: start_command: /usr/local/opt/git/libexec/git-core/git fsmonitor--daemon run --detach --ipc-threads=8

# Ctrl-C

$ rm -rf jj-fzf

# disabling fsmonitor fixes this, though

$ git config set --global core.fsmonitor false

$ git clone --no-checkout git@github.com:tim-janik/jj-fzf.git
Cloning into 'jj-fzf'...
remote: Enumerating objects: 1525, done.
remote: Counting objects: 100% (276/276), done.
remote: Compressing objects: 100% (74/74), done.
remote: Total 1525 (delta 264), reused 207 (delta 202), pack-reused 1249 (from 2)
Receiving objects: 100% (1525/1525), 307.26 KiB | 3.30 MiB/s, done.
Resolving deltas: 100% (922/922), done.

$ LC_ALL=C GIT_TRACE=1 /usr/local/bin/git --git-dir /Users/dougstephen/git/jj-fzf/.git fetch --verbose --prune --no-write-fetch-head --progress -- origin '+refs/heads/*:refs/remotes/origin/*'
11:10:40.163795 git.c:476               trace: built-in: git fetch --verbose --prune --no-write-fetch-head --progress -- origin '+refs/heads/*:refs/remotes/origin/*'
11:10:40.168012 run-command.c:667       trace: run_command: unset GIT_DIR GIT_PREFIX; GIT_PROTOCOL=version=2 ssh -o SendEnv=GIT_PROTOCOL git@github.com 'git-upload-pack '\''tim-janik/jj-fzf.git'\'''
11:10:40.168105 run-command.c:759       trace: start_command: /usr/bin/ssh -o SendEnv=GIT_PROTOCOL git@github.com 'git-upload-pack '\''tim-janik/jj-fzf.git'\'''
11:10:40.397812 run-command.c:667       trace: run_command: git rev-list --objects --stdin --not --exclude-hidden=fetch --all --quiet --alternate-refs
11:10:40.397869 run-command.c:759       trace: start_command: /usr/local/opt/git/libexec/git-core/git rev-list --objects --stdin --not --exclude-hidden=fetch --all --quiet --alternate-refs
From github.com:tim-janik/jj-fzf
 = [up to date]      jj_change_id -> origin/jj_change_id
 = [up to date]      next         -> origin/next
 = [up to date]      trunk        -> origin/trunk
11:10:40.455224 run-command.c:1535      run_processes_parallel: preparing to run up to 1 tasks
11:10:40.455271 run-command.c:1562      run_processes_parallel: done
11:10:40.455294 run-command.c:667       trace: run_command: git maintenance run --auto --no-quiet --detach
11:10:40.455354 run-command.c:759       trace: start_command: /usr/local/opt/git/libexec/git-core/git maintenance run --auto --no-quiet --detach
11:10:40.466861 git.c:476               trace: built-in: git maintenance run --auto --no-quiet --detach