Re: [PATCH v8 09/10] grep: simplify config parsing and option parsing

> When "grep.patternType" was introduced in 84befcd0a4a (grep: add a
> grep.patternType configuration setting, 2012-08-03) we promised that:
>
>  1. You can set "grep.patternType", and "[setting it to] 'default'
>     will return to the default matching behavior".
>
>     In that context "the default" meant whatever the configuration
>     system specified before that change, i.e. via grep.extendedRegexp.
>
>  2. We'd support the existing "grep.extendedRegexp" option, but ignore
>     it when the new "grep.patternType" option is set. We said we'd
>     only ignore the older "grep.extendedRegexp" option "when the
>     `grep.patternType` option is set. to a value other than
>     'default'".

> As before both "grep.patternType" and "grep.extendedRegexp" are
> last-one-wins variable, with "grep.extendedRegexp" yielding to
> "grep.patternType", except when "grep.patternType=default".
>
> Note that this applies as we parse the config, i.e. a sequence of:
>
>     -c grep.patternType=perl
>     -c grep.extendedRegexp=true \
>     -c grep.patternType=default
>
> Should select ERE due to "grep.extendedRegexp=true and

> because:
>
>  * If we see "grep.extendedRegexp" we set the internal "ero" to its
>    boolean value.
>
>  * If we see "grep.extendedRegexp" but
>    "grep.patternType=[default|<unset>]" is in effect we *don't* set
>    the internal "pattern_type_option" to update the pattern type.
>
>  * If we see "grep.patternType!=default" we can set our internal
>    "pattern_type_option" directly, it doesn't matter what the state of
>    "grep.extendedRegexp" is, but we don't forget what it was, in case
>    we see a "grep.patternType=default" again.
>
>  * If we see a "grep.patternType=default" we can set the pattern to
>    ERE or BRE depending on whether we last saw a
>    "grep.extendedRegexp=true" or
>    "grep.extendedRegexp=[false|<unset>]".

> diff --git a/grep.c b/grep.c
> index 60228a95a4f..bb487e994d0 100644
> --- a/grep.c
> +++ b/grep.c
> @@ -48,6 +48,12 @@ static int parse_pattern_type_arg(const char *opt, const char *arg)
>  
>  define_list_config_array_extra(color_grep_slots, {"match"});
>  
> +static void adjust_pattern_type(enum grep_pattern_type *pto, const int ero)
> +{
> +	if (*pto == GREP_PATTERN_TYPE_UNSPECIFIED)
> +		*pto = ero ? GREP_PATTERN_TYPE_ERE : GREP_PATTERN_TYPE_BRE;
> +}
> +
>  /*
>   * Read the configuration file once and store it in
>   * the grep_defaults template.
> @@ -56,17 +62,22 @@ int grep_config(const char *var, const char *value, void *cb)
>  {
>  	struct grep_opt *opt = cb;
>  	const char *slot;
> +	static int ero = -1;