feat: Add Repository::has_object() as a high-level alternative.

Previously, one would have to call `repo.objects.contains()`, which
is fine, but this method is necessary for symmetry of the API
and one shouldn't have to drop down a level to do this.

This method also knows empty trees as special case.

gix/src/repository/object.rs

@@ -26,7 +26,7 @@ impl crate::Repository {
     #[momo]
     pub fn find_object(&self, id: impl Into<ObjectId>) -> Result<Object<'_>, object::find::existing::Error> {
         let id = id.into();
-        if id == gix_hash::ObjectId::empty_tree(self.object_hash()) {
+        if id == ObjectId::empty_tree(self.object_hash()) {
             return Ok(Object {
                 id,
                 kind: gix_object::Kind::Tree,
@@ -46,7 +46,7 @@ impl crate::Repository {
     #[momo]
     pub fn find_header(&self, id: impl Into<ObjectId>) -> Result<gix_odb::find::Header, object::find::existing::Error> {
         let id = id.into();
-        if id == gix_hash::ObjectId::empty_tree(self.object_hash()) {
+        if id == ObjectId::empty_tree(self.object_hash()) {
             return Ok(gix_odb::find::Header::Loose {
                 kind: gix_object::Kind::Tree,
                 size: 0,
@@ -55,6 +55,25 @@ impl crate::Repository {
         self.objects.header(id)
     }
 
+    /// Return `true` if `id` exists in the object database.
+    ///
+    /// # Performance
+    ///
+    /// This method can be slow if the underlying [object database](crate::Repository::objects) has
+    /// an unsuitable [RefreshMode](gix_odb::store::RefreshMode) and `id` is not likely to exist.
+    /// Use [`repo.objects.refresh_never()`](gix_odb::store::Handle::refresh_never) to avoid expensive
+    /// IO-bound refreshes if an object wasn't found.
+    #[doc(alias = "exists", alias = "git2")]
+    #[momo]
+    pub fn has_object(&self, id: impl AsRef<gix_hash::oid>) -> bool {
+        let id = id.as_ref();
+        if id == ObjectId::empty_tree(self.object_hash()) {
+            true
+        } else {
+            self.objects.contains(id)
+        }
+    }
+
     /// Obtain information about an object without fully decoding it, or `None` if the object doesn't exist.
     ///
     /// Note that despite being cheaper than [`Self::try_find_object()`], there is still some effort traversing delta-chains.
@@ -64,7 +83,7 @@ impl crate::Repository {
         id: impl Into<ObjectId>,
     ) -> Result<Option<gix_odb::find::Header>, object::find::Error> {
         let id = id.into();
-        if id == gix_hash::ObjectId::empty_tree(self.object_hash()) {
+        if id == ObjectId::empty_tree(self.object_hash()) {
             return Ok(Some(gix_odb::find::Header::Loose {
                 kind: gix_object::Kind::Tree,
                 size: 0,
@@ -77,7 +96,7 @@ impl crate::Repository {
     #[momo]
     pub fn try_find_object(&self, id: impl Into<ObjectId>) -> Result<Option<Object<'_>>, object::find::Error> {
         let id = id.into();
-        if id == gix_hash::ObjectId::empty_tree(self.object_hash()) {
+        if id == ObjectId::empty_tree(self.object_hash()) {
             return Ok(Some(Object {
                 id,
                 kind: gix_object::Kind::Tree,
@@ -236,7 +255,7 @@ impl crate::Repository {
         reference: FullName,
         message: &str,
         tree: ObjectId,
-        parents: SmallVec<[gix_hash::ObjectId; 1]>,
+        parents: SmallVec<[ObjectId; 1]>,
     ) -> Result<Id<'_>, commit::Error> {
         use gix_ref::{
             transaction::{Change, RefEdit},
@@ -310,12 +329,12 @@ impl crate::Repository {
         self.commit_as(committer, author, reference, message, tree, parents)
     }
 
-    /// Return an empty tree object, suitable for [getting changes](crate::Tree::changes()).
+    /// Return an empty tree object, suitable for [getting changes](Tree::changes()).
     ///
     /// Note that the returned object is special and doesn't necessarily physically exist in the object database.
     /// This means that this object can be used in an uninitialized, empty repository which would report to have no objects at all.
     pub fn empty_tree(&self) -> Tree<'_> {
-        self.find_object(gix_hash::ObjectId::empty_tree(self.object_hash()))
+        self.find_object(ObjectId::empty_tree(self.object_hash()))
             .expect("always present")
             .into_tree()
     }

gix/tests/repository/object.rs

@@ -68,6 +68,7 @@ fn writes_avoid_io_using_duplicate_check() -> crate::Result {
 
     for id in repo.objects.iter()? {
         let id = id?;
+        assert!(repo.has_object(id));
         let obj = repo.find_object(id)?;
         let header = repo.find_header(id)?;
         assert_eq!(obj.kind, header.kind(), "header and object agree");
@@ -156,6 +157,7 @@ mod find {
         let repo = basic_repo()?;
         let empty_tree = gix::hash::ObjectId::empty_tree(repo.object_hash());
         assert_eq!(repo.find_object(empty_tree)?.into_tree().iter().count(), 0);
+        assert!(repo.has_object(empty_tree));
         assert_eq!(
             repo.find_header(empty_tree)?,
             gix_odb::find::Header::Loose {