rust-repo: move dirstate-v2 opening to a separate method

The next changeset will make changes to this logic, it helps to have it
in order first.

rust/hg-core/src/repo.rs

@@ -305,14 +305,31 b' impl Repo {'
     }
 
     fn new_dirstate_map(&self) -> Result<OwningDirstateMap, DirstateError> {
+        if self.has_dirstate_v2() {
+            self.read_docket_and_data_file()
+        } else {
         let dirstate_file_contents = self.dirstate_file_contents()?;
         if dirstate_file_contents.is_empty() {
             self.dirstate_parents.set(DirstateParents::NULL);
-            if self.has_dirstate_v2() {
-                self.dirstate_data_file_uuid.set(None);
+                Ok(OwningDirstateMap::new_empty(Vec::new()))
+            } else {
+                let (map, parents) =
+                    OwningDirstateMap::new_v1(dirstate_file_contents)?;
+                self.dirstate_parents.set(parents);
+                Ok(map)
+            }
+        }
             }
-            Ok(OwningDirstateMap::new_empty(Vec::new()))
-        } else if self.has_dirstate_v2() {
+
+    fn read_docket_and_data_file(
+        &self,
+    ) -> Result<OwningDirstateMap, DirstateError> {
+        let dirstate_file_contents = self.dirstate_file_contents()?;
+        if dirstate_file_contents.is_empty() {
+            self.dirstate_parents.set(DirstateParents::NULL);
+            self.dirstate_data_file_uuid.set(None);
+            return Ok(OwningDirstateMap::new_empty(Vec::new()));
+        }
             let docket = crate::dirstate_tree::on_disk::read_docket(
                 &dirstate_file_contents,
             )?;
@@ -321,8 +338,7 b' impl Repo {'
                 .set(Some(docket.uuid.to_owned()));
             let data_size = docket.data_size();
             let metadata = docket.tree_metadata();
-            let mut map =
-                if crate::vfs::is_on_nfs_mount(docket.data_filename()) {
+        let mut map = if crate::vfs::is_on_nfs_mount(docket.data_filename()) {
                     // Don't mmap on NFS to prevent `SIGBUS` error on deletion
                     OwningDirstateMap::new_v2(
                         self.hg_vfs().read(docket.data_filename())?,
@@ -354,12 +370,6 b' impl Repo {'
             map.with_dmap_mut(|m| m.set_write_mode(write_mode));
 
             Ok(map)
-        } else {
-            let (map, parents) =
-                OwningDirstateMap::new_v1(dirstate_file_contents)?;
-            self.dirstate_parents.set(parents);
-            Ok(map)
-        }
     }
 
     pub fn dirstate_map(