git: use force branch checkout to prevent cases when .gitattributes modifies

files by specific rule and then checkout is blocked by potential file conflicts.

rhodecode/lib/vcs/backends/git/repository.py

@@ -900,10 +900,10 b' class GitRepository(BaseRepository):'
         # fetch proper commits for merge testing
         if source_ref.name != target_ref.name:
             if shadow_repo.get_remote_ref(source_ref.name):
-                shadow_repo._checkout(source_ref.name)
+                shadow_repo._checkout(source_ref.name, force=True)
 
         # checkout target
-        shadow_repo._checkout(target_ref.name)
+        shadow_repo._checkout(target_ref.name, force=True)
         shadow_repo._local_pull(self.path, target_ref.name)
 
         # Need to reload repo to invalidate the cache, or otherwise we cannot