branch: stop modifying `log_all_ref_updates` variable

In "branch.c" we modify the global `log_all_ref_updates` variable to force creation of a reflog entry. Modifying global state like this is discouraged, as it may have all kinds of consequences in other places of our codebase. Stop modifying the variable and pass the `REF_FORCE_CREATE_REFLOG` flag instead. Setting this flag has a stronger meaning than setting the config to `LOG_REFS_NORMAL`: - `LOG_REFS_NORMAL` will ask us to only create reflog entries for preexisting reflogs or branches, remote refs, note refs and HEAD. - `REF_FORCE_CREATE_REFLOG` will unconditionally create a reflog and is thus equivalent to `LOG_REFS_ALWAYS`. But as we are in `create_branch()` and thus do not have to worry about arbitrary references, but only about branches, `LOG_REFS_NORMAL` and `LOG_REFS_ALWAYS` are indeed equivalent. Signed-off-by: Patrick Steinhardt <ps@pks.im> Signed-off-by: Junio C Hamano <gitster@pobox.com>