fix(git): convert default_branch to full ref name (refs/heads/*)

The database stores short branch names (e.g., "main"), but git2's push_ref()
requires full reference names (e.g., "refs/heads/main"). This fixes all
service-layer endpoints to convert default_branch to the full ref format.

Fixed endpoints:
- git_readme: convert to refs/heads/{branch}
- git_commit_count: convert to refs/heads/{branch}
- git_contributors: convert to refs/heads/{branch}
- git_commit_log: convert to refs/heads/{branch}
- git_commit_walk: convert to refs/heads/{branch}

Resolves errors:
- Internal("the given reference name 'main' is not valid")

libs/service/git/blob.rs

@@ -133,7 +133,13 @@ impl AppService {
         let repo = self
             .utils_find_repo(namespace.clone(), repo_name.clone(), ctx)
             .await?;
-        let rev = query.r#ref.unwrap_or_else(|| repo.default_branch.clone());
+        let rev = query.r#ref.unwrap_or_else(|| {
+            if repo.default_branch.is_empty() {
+                "HEAD".to_string()
+            } else {
+                format!("refs/heads/{}", repo.default_branch)
+            }
+        });
 
         let tree_oid: git::CommitOid = {
             let rev_clone = rev.clone();

libs/service/git/commit.rs

@@ -786,7 +786,13 @@ impl AppService {
         let repo = self
             .utils_find_repo(namespace.clone(), repo_name.clone(), ctx)
             .await?;
-        let rev_clone = query.rev.clone().or_else(|| Some(repo.default_branch.clone()));
+        let rev_clone = query.rev.clone().or_else(|| {
+            if repo.default_branch.is_empty() {
+                None
+            } else {
+                Some(format!("refs/heads/{}", repo.default_branch))
+            }
+        });
         let rev_for_count = rev_clone.clone();
 
         let commits = git_spawn!(repo, domain -> {
@@ -853,11 +859,15 @@ impl AppService {
         let repo = self.utils_find_repo(namespace, repo_name, ctx).await?;
         let from_clone = from.clone();
         let to_clone = to.clone();
-        let default_branch = repo.default_branch.clone();
+        let default_ref = if repo.default_branch.is_empty() {
+            None
+        } else {
+            Some(format!("refs/heads/{}", repo.default_branch))
+        };
 
         let count = git_spawn!(repo, domain -> {
             let from_ref = from_clone.as_deref();
-            let to_ref = to_clone.as_deref().or(Some(default_branch.as_str()));
+            let to_ref = to_clone.as_deref().or(default_ref.as_deref());
             domain.commit_count(from_ref, to_ref)
         })?;
 
@@ -1276,7 +1286,13 @@ impl AppService {
         }
 
         let repo = self.utils_find_repo(namespace, repo_name, ctx).await?;
-        let rev_clone = query.rev.clone();
+        let rev_clone = query.rev.clone().or_else(|| {
+            if repo.default_branch.is_empty() {
+                None
+            } else {
+                Some(format!("refs/heads/{}", repo.default_branch))
+            }
+        });
         let limit = query.limit.unwrap_or(0);
         let first_parent_only = query.first_parent_only;
         let topological = query.topological;

libs/service/git/contributors.rs

@@ -65,7 +65,13 @@ impl AppService {
         }
 
         let repo_clone = repo.clone();
-        let ref_name_clone = query.ref_name.clone().or_else(|| Some(repo.default_branch.clone()));
+        let ref_name_clone = query.ref_name.clone().or_else(|| {
+            if repo.default_branch.is_empty() {
+                None
+            } else {
+                Some(format!("refs/heads/{}", repo.default_branch))
+            }
+        });
 
         let commits = tokio::task::spawn_blocking(move || {
             let domain = git::GitDomain::from_model(repo_clone)?;