for-each-ref: delay parsing of --sort=<atom> options

The for-each-ref family of commands invoke parsers immediately when it sees each --sort=<atom> option, and die before even seeing the other options on the command line when the <atom> is unrecognised. Instead, accumulate them in a string list, and have them parsed into a ref_sorting structure after the command line parsing is done. As a consequence, "git branch --sort=bogus -h" used to fail to give the brief help, which arguably may have been a feature, now does so, which is more consistent with how other options work. The patch is smaller than the actual extent of the "damage" to the codebase, thanks to the fact that the original code consistently used OPT_REF_SORT() macro to handle command line options. We only needed to replace the variable used for the list, and implementation of the callback function used in the macro. The old rule was for the users of the API to: - Declare ref_sorting and ref_sorting_tail variables; - OPT_REF_SORT() macro will instantiate ref_sorting instance (which may barf and die) and append it to the tail; - Append to the tail each ref_sorting read from the configuration by parsing in the config callback (which may barf and die); - See if ref_sorting is null and use ref_sorting_default() instead. Now the rule is not all that different but is simpler: - Declare ref_sorting_options string list. - OPT_REF_SORT() macro will append it to the string list; - Append to the string list the sort key read from the configuration; - call ref_sorting_options() to turn the string list to ref_sorting structure (which also deals with the default value). As side effects, this change also cleans up a few issues: - 95be717c (parse_opt_ref_sorting: always use with NONEG flag, 2019-03-20) muses that "git for-each-ref --no-sort" should simply clear the sort keys accumulated so far; it now does. - The implementation detail of "struct ref_sorting" and the helper function parse_ref_sorting() can now be private to the ref-filter API implementation. - If you set branch.sort to a bogus value, the any "git branch" invocation, not only the listing mode, would abort with the original code; now it doesn't Signed-off-by: Junio C Hamano <gitster@pobox.com>