The same as above but lookup by name.
-If given the null_sha1 as treeish_name the local configuration of a
-submodule will be returned (e.g. consolidated values from local git
+Whenever a submodule configuration is parsed in `parse_submodule_config_option`
+via e.g. `gitmodules_config()`, it will overwrite the null_sha1 entry.
+So in the normal case, when HEAD:.gitmodules is parsed first and then overlayed
+with the repository configuration, the null_sha1 entry contains the local
+configuration of a submodule (e.g. consolidated values from local git
configuration and the .gitmodules file in the worktree).
For an example usage see test-submodule-config.c.