fix: Repository::tree_merge_options() now comes with rewrite tracking.

This is the way Git acts, as it's either configured, or defaults to the value
coming from the `diff.renames` configuration.

gix/src/diff.rs

@@ -175,7 +175,7 @@ pub(crate) mod utils {
         new_rewrites_inner(config, lenient, &Diff::RENAMES, &Diff::RENAME_LIMIT)
     }
 
-    pub fn new_rewrites_inner(
+    pub(crate) fn new_rewrites_inner(
         config: &gix_config::File<'static>,
         lenient: bool,
         renames: &'static crate::config::tree::diff::Renames,

gix/src/repository/merge.rs

@@ -89,12 +89,20 @@ impl Repository {
     /// Read all relevant configuration options to instantiate options for use in [`merge_trees()`](Self::merge_trees).
     pub fn tree_merge_options(&self) -> Result<crate::merge::tree::Options, tree_merge_options::Error> {
         Ok(gix_merge::tree::Options {
-            rewrites: crate::diff::utils::new_rewrites_inner(
-                &self.config.resolved,
-                self.config.lenient_config,
-                &tree::Merge::RENAMES,
-                &tree::Merge::RENAME_LIMIT,
-            )?,
+            rewrites: Some(
+                crate::diff::utils::new_rewrites_inner(
+                    &self.config.resolved,
+                    self.config.lenient_config,
+                    &tree::Merge::RENAMES,
+                    &tree::Merge::RENAME_LIMIT,
+                )?
+                .map(Ok)
+                .or_else(|| {
+                    crate::diff::utils::new_rewrites(&self.config.resolved, self.config.lenient_config).transpose()
+                })
+                .transpose()?
+                .unwrap_or_default(),
+            ),
             blob_merge: self.blob_merge_options()?,
             blob_merge_command_ctx: self.command_context()?,
             fail_on_conflict: None,