repo-upgrade: write new requirement before upgrading the dirstate

This will prevent a small race condition where another hg process still
believes the repo is dirstate-v1 during the upgrade process.

This is good to have, but it is not a proper fix for the underlying problem.
There is code that assumes a requirement means a usage, e.g. having the
`generaldelta` requirement would imply *all* revlogs to use general delta,
but it's not true, it simply means that the repository advertises to the
client it needs to understand `generaldelta` in order to read the repo.

In the case of the dirstate, having the requirement *technically* should always
be the same as using dirstate-v2, since there is only one dirstate and
requirements should be as minimal as possible. However, we should not assume
this and make the code more robust in a future patch (series).

mercurial/upgrade_utils/engine.py

@@ -655,9 +655,14 b' def upgrade_dirstate(ui, srcrepo, upgrad'
             pass
 
     assert srcrepo.dirstate._use_dirstate_v2 == (old == b'v2')
+    use_v2 = new == b'v2'
+    if use_v2:
+        # Write the requirements *before* upgrading
+        scmutil.writereporequirements(srcrepo, upgrade_op.new_requirements)
+
     srcrepo.dirstate._map.preload()
-    srcrepo.dirstate._use_dirstate_v2 = new == b'v2'
-    srcrepo.dirstate._map._use_dirstate_v2 = srcrepo.dirstate._use_dirstate_v2
+    srcrepo.dirstate._use_dirstate_v2 = use_v2
+    srcrepo.dirstate._map._use_dirstate_v2 = use_v2
     srcrepo.dirstate._dirty = True
     try:
         srcrepo.vfs.unlink(b'dirstate')
@@ -667,8 +672,9 b' def upgrade_dirstate(ui, srcrepo, upgrad'
         pass
 
     srcrepo.dirstate.write(None)
-
-    scmutil.writereporequirements(srcrepo, upgrade_op.new_requirements)
+    if not use_v2:
+        # Remove the v2 requirement *after* downgrading
+        scmutil.writereporequirements(srcrepo, upgrade_op.new_requirements)
 
 
 def upgrade_tracked_hint(ui, srcrepo, upgrade_op, add):