Code clean-up and duplicate removal.
* bw/submodule-has-commits-update:
submodule: refactor logic to determine changed submodules
submodule: improve submodule_has_commits()
submodule: change string_list changed_submodule_paths
submodule: remove add_oid_to_argv()
submodule: rename free_submodules_sha1s()
submodule: rename add_sha1_to_array()