Don't deviate by creating strange reflogs (with null-source & null-destination) (#450)

That's exactly what git does, so it's probably the right thing to do if
in doubt.

Author: Byron

git-repository/src/clone/fetch/mod.rs

@@ -41,11 +41,6 @@ impl PrepareFetch {
     /// it was newly initialized.
     ///
     /// Note that all data we created will be removed once this instance drops if the operation wasn't successful.
-    ///
-    /// # Deviation
-    ///
-    /// When the remote side is freshly initialized without commits, we pick up their reference name _and_ create a reflog entry like
-    /// before, with old and new hash being the `null-hex-sha`. That way the branch still remembers where it was created from.
     #[cfg(feature = "blocking-network-client")]
     pub fn fetch_only<P>(
         &mut self,

git-repository/src/clone/fetch/util.rs

@@ -117,20 +117,19 @@ pub fn update_head(
                 .commit(repo.committer_or_default())
                 .map_err(crate::reference::edit::Error::from)?;
 
-            let mut log = reflog_message();
-            log.mode = RefLog::Only;
-            repo.edit_reference(RefEdit {
-                change: git_ref::transaction::Change::Update {
-                    log,
-                    expected: PreviousValue::Any,
-                    new: Target::Peeled(match head_peeled_id {
-                        Some(id) => id.to_owned(),
-                        None => git_hash::ObjectId::null(repo.object_hash()),
-                    }),
-                },
-                name: head,
-                deref: false,
-            })?;
+            if let Some(head_peeled_id) = head_peeled_id {
+                let mut log = reflog_message();
+                log.mode = RefLog::Only;
+                repo.edit_reference(RefEdit {
+                    change: git_ref::transaction::Change::Update {
+                        log,
+                        expected: PreviousValue::Any,
+                        new: Target::Peeled(*head_peeled_id),
+                    },
+                    name: head,
+                    deref: false,
+                })?;
+            }
         }
         None => {
             repo.edit_reference(RefEdit {

git-repository/tests/clone/mod.rs

@@ -208,8 +208,10 @@ mod blocking_io {
         let head = repo.head()?;
         assert!(head.is_unborn());
 
-        let mut logs = head.log_iter();
-        assert_reflog(logs.all());
+        assert!(
+            head.log_iter().all()?.is_none(),
+            "no reflog for unborn heads (as it needs non-null destination hash)"
+        );
 
         if out
             .ref_map