dirstate: do not write an empty dirstate just for backup

This will get in the way when we get more strict about holding the lock when
writing the dirstate.

Instead, we simply don't copy dirstate files around if there are None at backup
time.

A couple of tests are impacted they no longer need to backup such "empty"
dirstate.

mercurial/dirstate.py

@@ -1486,6 +1486,9 b' class dirstate:'
         """return a filename to backup a data-file or None"""
         if not self._use_dirstate_v2:
             return None
+        if self._map.docket.uuid is None:
+            # not created yet, nothing to backup
+            return None
         data_filename = self._map.docket.data_filename()
         return data_filename, self.data_backup_filename(backupname)
 
@@ -1506,7 +1509,7 b' class dirstate:'
         # use '_writedirstate' instead of 'write' to write changes certainly,
         # because the latter omits writing out if transaction is running.
         # output file will be used to create backup of dirstate at this point.
-        if self._dirty or not self._opener.exists(filename):
+        if self._dirty:
             self._writedirstate(
                 tr,
                 self._opener(filename, b"w", atomictemp=True, checkambig=True),
@@ -1530,7 +1533,7 b' class dirstate:'
             tr.registertmp(filename, location=b'plain')
 
         self._opener.tryunlink(backupname)
-        if True:
+        if self._opener.exists(filename):
             # hardlink backup is okay because _writedirstate is always called
             # with an "atomictemp=True" file.
             util.copyfile(
@@ -1557,10 +1560,23 b' class dirstate:'
         # this "invalidate()" prevents "wlock.release()" from writing
         # changes of dirstate out after restoring from backup file
         self.invalidate()
+        o = self._opener
+        if not o.exists(backupname):
+            # there was no file backup, delete existing files
+            filename = self._actualfilename(tr)
+            data_file = None
+            if self._use_dirstate_v2:
+                data_file = self._map.docket.data_filename()
+            if o.exists(filename):
+                o.unlink(filename)
+            if data_file is not None and o.exists(data_file):
+                o.unlink(data_file)
+            return
         filename = self._actualfilename(tr)
-        o = self._opener
         data_pair = self.backup_data_file(backupname)
-        if util.samefile(o.join(backupname), o.join(filename)):
+        if o.exists(filename) and util.samefile(
+            o.join(backupname), o.join(filename)
+        ):
             o.unlink(backupname)
         else:
             o.rename(backupname, filename, checkambig=True)
@@ -1577,11 +1593,11 b' class dirstate:'
     def clearbackup(self, tr, backupname):
         '''Clear backup file'''
         o = self._opener
-        data_backup = self.backup_data_file(backupname)
-        o.unlink(backupname)
-
-        if data_backup is not None:
-            o.unlink(data_backup[0])
+        if o.exists(backupname):
+            data_backup = self.backup_data_file(backupname)
+            o.unlink(backupname)
+            if data_backup is not None:
+                o.unlink(data_backup[0])
 
     def verify(self, m1, m2, p1, narrow_matcher=None):
         """

tests/test-empty.t

@@ -36,7 +36,6 b' Poke at a clone:'
   $ ls .hg
   00changelog.i
   cache
-  dirstate
   hgrc
   requires
   store

tests/test-inherit-mode.t

@@ -137,7 +137,6 b' group can still write everything'
   00660 ../push/.hg/cache/branch2-base
   00660 ../push/.hg/cache/rbc-names-v1
   00660 ../push/.hg/cache/rbc-revs-v1
-  00660 ../push/.hg/dirstate
   00660 ../push/.hg/requires
   00770 ../push/.hg/store/
   00660 ../push/.hg/store/00changelog.i
@@ -160,7 +159,6 b' group can still write everything'
   00660 ../push/.hg/undo.bookmarks
   00660 ../push/.hg/undo.branch
   00660 ../push/.hg/undo.desc
-  00660 ../push/.hg/undo.dirstate
   00770 ../push/.hg/wcache/