submodule API: don't handle SM_..{UNSPECIFIED,COMMAND} in to_string()

Change the submodule_strategy_to_string() function added in 3604242f080 (submodule: port init from shell to C, 2016-04-15) to really return a "const char *". In the "SM_UPDATE_COMMAND" case it would return a strbuf_detach(). Furthermore, this function would return NULL on SM_UPDATE_UNSPECIFIED, so it wasn't safe to xstrdup() its return value in the general case, or to use it in a sprintf() format as the code removed in the preceding commit did. But its callers would never call it with either SM_UPDATE_UNSPECIFIED or SM_UPDATE_COMMAND. Let's have its behavior reflect how its only user expects it to behave, and BUG() out on the rest. By doing this we can also stop needlessly xstrdup()-ing and free()-ing the memory for the config we're setting. We can instead always use constant strings. We can also use the *_tmp() variant of git_config_get_string(). Let's also rename this submodule_strategy_to_string() function to submodule_update_type_to_string(). Now that it's only tasked with returning a string version of the "enum submodule_update_type type". Before it would look at the "command" field in "struct submodule_update_strategy". Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com> Reviewed-by: Glen Choo <chooglen@google.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>