Hooray! My intuition was right (portable dumper issue):

```
root@d71b23f3596a:/opt/emacs-git# git bisect start master emacs-26.1.91
root@d71b23f3596a:/opt/emacs-git# git bisect run bash -c 'git reset --hard;
make -j8 &>/dev/null || exit 125; src/emacs -f save-buffers-kill-emacs
&>/dev/null || exit 1'
(...snip...)
d12e5d003d503025c1c9b0335d6518a6c3bdfae1 is the first bad commit
commit d12e5d003d503025c1c9b0335d6518a6c3bdfae1
Author: Daniel Colascione <dancol@dancol.org>
Date:   Tue Jan 15 17:36:54 2019 -0500

    Add portable dumper
(...snip...)

root@d71b23f3596a:/opt/emacs-git# git bisect log
# bad: [19fbef549a94ccf733367d29438204e94a00e911] Fix Bug#34196
# good: [d8525ae41d07f9ea629d610de791064180423b6a] Bump Emacs version to
26.1.91
git bisect start 'master' 'emacs-26.1.91'
# good: [aaffae8458dcd774540e7e6b4219c8b5a9902075] Add debug facility for
formatting in rr sessions
git bisect good aaffae8458dcd774540e7e6b4219c8b5a9902075
# good: [a0605d96187bc4103a982cededcd12e2628aba66] Fix MinGW compilation
problem in timefns.c
git bisect good a0605d96187bc4103a982cededcd12e2628aba66
# good: [34b4da377ae02a0c505574f5ca5f146e92cfd046] Fix an eshell ls dired
test for non-recent files
git bisect good 34b4da377ae02a0c505574f5ca5f146e92cfd046
# good: [34b4da377ae02a0c505574f5ca5f146e92cfd046] Fix an eshell ls dired
test for non-recent files
git bisect good 34b4da377ae02a0c505574f5ca5f146e92cfd046
# bad: [655badc33e6ee9bfbc6c6c9084bf768f8102824d] ; Copyright fixes for
pdumper files
git bisect bad 655badc33e6ee9bfbc6c6c9084bf768f8102824d
# good: [fb10834a602416f8422131d5ce9dabcc28e57be4] Avoid that
unwind_format_mode_line messes up buffer points (Bug#32777)
git bisect good fb10834a602416f8422131d5ce9dabcc28e57be4
# good: [517b0aa46663b6173bb31a018aee18a82f2ca1d9] Merge from
origin/emacs-26
git bisect good 517b0aa46663b6173bb31a018aee18a82f2ca1d9
# good: [c342b26371480316024e1e5d63cd8b3f035dda69] Fix drag and drop
behaviour on NS (bug#30929)
git bisect good c342b26371480316024e1e5d63cd8b3f035dda69
# good: [cdb082322d4209c5104bc1a98b21bf3dd75e8f17] Fix icomplete's cycling
when filename filtering kicks in
git bisect good cdb082322d4209c5104bc1a98b21bf3dd75e8f17
# good: [2a3bd6798e9670828f0402079fcc116d6d6b042d] Avoid using obsolete
accept-process-output arg
git bisect good 2a3bd6798e9670828f0402079fcc116d6d6b042d
# bad: [6b9fa8804533a695094a930d634d2d6617e2b6c7] Make sure dump-mode is
nil after dump
git bisect bad 6b9fa8804533a695094a930d634d2d6617e2b6c7
# bad: [02976d67369699660add46d548f0d1593885334b] Add NEWS for pdumper
git bisect bad 02976d67369699660add46d548f0d1593885334b
# bad: [d12e5d003d503025c1c9b0335d6518a6c3bdfae1] Add portable dumper
git bisect bad d12e5d003d503025c1c9b0335d6518a6c3bdfae1
# first bad commit: [d12e5d003d503025c1c9b0335d6518a6c3bdfae1] Add portable
dumper
```

I quickly looked at the pdumper commit for clues but couldn't find any.

Kind regards,
Philippe