Merge branch 'ab/submodule-helper-prep'

Merge branch 'ab/submodule-helper-prep'

Code clean-up of "git submodule--helper".

* ab/submodule-helper-prep: (33 commits)
  submodule--helper: fix bad config API usage
  submodule--helper: libify even more "die" paths for module_update()
  submodule--helper: libify more "die" paths for module_update()
  submodule--helper: check repo{_submodule,}_init() return values
  submodule--helper: libify "must_die_on_failure" code paths (for die)
  submodule--helper update: don't override 'checkout' exit code
  submodule--helper: libify "must_die_on_failure" code paths
  submodule--helper: libify determine_submodule_update_strategy()
  submodule--helper: don't exit() on failure, return
  submodule--helper: use "code" in run_update_command()
  submodule API: don't handle SM_..{UNSPECIFIED,COMMAND} in to_string()
  submodule--helper: don't call submodule_strategy_to_string() in BUG()
  submodule--helper: add missing braces to "else" arm
  submodule--helper: return "ret", not "1" from update_submodule()
  submodule--helper: rename "int res" to "int ret"
  submodule--helper: don't redundantly check "else if (res)"
  submodule--helper: refactor "errmsg_str" to be a "struct strbuf"
  submodule--helper: add "const" to passed "struct update_data"
  submodule--helper: add "const" to copy of "update_data"
  submodule--helper: add "const" to passed "module_clone_data"
  ...