fetch: merge config instead of updating

Update will replace sections instead of merging them together, which
is not desirable here.

Related https://github.com/iterative/studio/pull/6541

dvc/config.py

@@ -165,6 +165,9 @@ class Config(dict):
         with open(config_file, "w+", encoding="utf-8"):
             return Config(dvc_dir)
 
+    def merge(self, config):
+        merge(self, config)
+
     def load(
         self,
         validate: bool = True,

dvc/repo/fetch.py

@@ -48,7 +48,7 @@ def _collect_indexes(  # noqa: PLR0913
         all_commits=all_commits,
     ):
         try:
-            repo.config.update(config)
+            repo.config.merge(config)
 
             idx = repo.index.targets_view(
                 targets,