doc/bugs/unusable_with_ghc_8.10.7_on_DragonFlyBSD.mdwn

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+On DragonflyBSD with git-annex built with ghc 8.10.7, a simple `git annex add` puts strange entries in the index, as shown:
+
+```
+copter-dfly tmp $ mkdir d
+copter-dfly tmp $ cd d
+copter-dfly d $ git init
+Initialized empty Git repository in /home/falsifian/tmp/d/.git/
+copter-dfly d $ git annex init
+init  ok
+(recording state in git...)
+copter-dfly d $ echo test>f
+copter-dfly d $ git annex add f
+add f
+ok
+(recording state in git...)
+copter-dfly d $ git status
+On branch master
+
+No commits yet
+
+Changes to be committed:
+  (use "git rm --cached <file>..." to unstage)
+        new file:   30d/f3a/SHA256-s5--f2ca1bb6c7e907d06dafe4687e579fce76b37e4e93b7605022da52e6ccc26fd2.log
+        new file:   f
+        new file:   uuid.log
+
+Changes not staged for commit:
+  (use "git add/rm <file>..." to update what will be committed)
+  (use "git restore <file>..." to discard changes in working directory)
+        deleted:    30d/f3a/SHA256-s5--f2ca1bb6c7e907d06dafe4687e579fce76b37e4e93b7605022da52e6ccc26fd2.log
+        deleted:    uuid.log
+
+copter-dfly d $
+```
+
+Digging into it further with dfly people over IRC, it's been discovered:
+
+* The bug does not appear with ghc 8.10.5, with git-annex 20210903 or 20210223.
+* But it consistently happens with ghc 8.10.7, with all the versions I tried.
+* I also noticed that `git annex init` does not create the file `.git/annex/index` when the bug is present.
+
+Has anyone tried git-annex with ghc 8.10.7 on a different OS?
+
+I tried to figure out why `.git/annex/index` was not being created, but I had some trouble figuring out which line of code is supposed to create it. If someone could point me there I could do some further debugging to see why it isn't created.