refactor(git): use DEFAULT_REVISION constant across git operations

- Replace 15 occurrences of unwrap_or("HEAD") with
  unwrap_or(DEFAULT_REVISION) across 10 files
- All git API handlers and service methods now reference the shared
  constant from models::common

api/repo/git/git_compare_branch.rs

@@ -0,0 +1,54 @@
+use crate::models::common::DEFAULT_REVISION;
+use actix_web::{HttpResponse, web};
+use serde::Deserialize;
+use utoipa::IntoParams;
+
+use crate::api::response::{ApiErrorResponse, ApiResponse};
+use crate::error::AppError;
+use crate::service::AppService;
+use crate::session::Session;
+
+#[derive(Debug, Deserialize, IntoParams)]
+pub struct PathParams {
+    pub workspace_name: String,
+    pub repo_name: String,
+    pub branch_name: String,
+}
+
+#[derive(Debug, Deserialize, IntoParams)]
+pub struct QueryParams {
+    pub base: Option<String>,
+}
+
+#[utoipa::path(
+    get,
+    path = "/api/v1/workspaces/{workspace_name}/repos/{repo_name}/git/branches/{branch_name}/compare",
+    tag = "Git",
+    operation_id = "gitCompareBranch",
+    params(PathParams, QueryParams),
+    responses(
+        (status = 200, description = "Branch comparison", body = ApiResponse<crate::pb::repo::CompareBranchResponse>),
+        (status = 401, description = "Unauthorized", body = ApiErrorResponse),
+        (status = 404, description = "Not found", body = ApiErrorResponse),
+    ),
+    security(("session_cookie" = []))
+)]
+pub async fn git_compare_branch(
+    service: web::Data<AppService>,
+    session: Session,
+    path: web::Path<PathParams>,
+    query: web::Query<QueryParams>,
+) -> Result<HttpResponse, AppError> {
+    let base = query.base.as_deref().unwrap_or(DEFAULT_REVISION);
+    let result = service
+        .repo
+        .git_compare_branches(
+            &session,
+            &path.workspace_name,
+            &path.repo_name,
+            &path.branch_name,
+            base,
+        )
+        .await?;
+    Ok(HttpResponse::Ok().json(ApiResponse::new(result)))
+}