ref-filter: add multiple-option parsing functions

The functions

	match_placeholder_arg_value()
	match_placeholder_bool_arg()

were added in pretty 4f732e0fd7 (pretty: allow %(trailers) options
with explicit value, 2019-01-29) to parse multiple options in an
argument to --pretty. For example,

	git log --pretty="%(trailers:key=Signed-Off-By,separator=%x2C )"

will output all the trailers matching the key and seperates them by
a comma followed by a space per commit.

Add similar functions,

	match_atom_arg_value()
	match_atom_bool_arg()

in ref-filter.

There is no atom yet that can use these functions in ref-filter, but we
are going to add a new %(describe) atom in a subsequent commit where we
parse options like tags=<bool-value> or match=<pattern> given to it.

Helped-by: Junio C Hamano <gitster@pobox.com>
Mentored-by: Christian Couder <christian.couder@gmail.com>
Mentored-by: Hariom Verma <hariom18599@gmail.com>
Signed-off-by: Kousik Sanagavarapu <five231003@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

ref-filter.c

@@ -1,6 +1,7 @@
 #include "git-compat-util.h"
 #include "environment.h"
 #include "gettext.h"
+#include "config.h"
 #include "gpg-interface.h"
 #include "hex.h"
 #include "parse-options.h"
@@ -255,6 +256,110 @@ static int err_bad_arg(struct strbuf *sb, const char *name, const char *arg)
 	return -1;
 }
 
+/*
+ * Parse option of name "candidate" in the option string "to_parse" of
+ * the form
+ *
+ *	"candidate1[=val1],candidate2[=val2],candidate3[=val3],..."
+ *
+ * The remaining part of "to_parse" is stored in "end" (if we are
+ * parsing the last candidate, then this is NULL) and the value of
+ * the candidate is stored in "valuestart" and its length in "valuelen",
+ * that is the portion after "=". Since it is possible for a "candidate"
+ * to not have a value, in such cases, "valuestart" is set to point to
+ * NULL and "valuelen" to 0.
+ *
+ * The function returns 1 on success. It returns 0 if we don't find
+ * "candidate" in "to_parse" or we find "candidate" but it is followed
+ * by more chars (for example, "candidatefoo"), that is, we don't find
+ * an exact match.
+ *
+ * This function only does the above for one "candidate" at a time. So
+ * it has to be called each time trying to parse a "candidate" in the
+ * option string "to_parse".
+ */
+static int match_atom_arg_value(const char *to_parse, const char *candidate,
+				const char **end, const char **valuestart,
+				size_t *valuelen)
+{
+	const char *atom;
+
+	if (!skip_prefix(to_parse, candidate, &atom))
+		return 0; /* definitely not "candidate" */
+
+	if (*atom == '=') {
+		/* we just saw "candidate=" */
+		*valuestart = atom + 1;
+		atom = strchrnul(*valuestart, ',');
+		*valuelen = atom - *valuestart;
+	} else if (*atom != ',' && *atom != '\0') {
+		/* key begins with "candidate" but has more chars */
+		return 0;
+	} else {
+		/* just "candidate" without "=val" */
+		*valuestart = NULL;
+		*valuelen = 0;
+	}
+
+	/* atom points at either the ',' or NUL after this key[=val] */
+	if (*atom == ',')
+		atom++;
+	else if (*atom)
+		BUG("Why is *atom not NULL yet?");
+
+	*end = atom;
+	return 1;
+}
+
+/*
+ * Parse boolean option of name "candidate" in the option list "to_parse"
+ * of the form
+ *
+ *	"candidate1[=bool1],candidate2[=bool2],candidate3[=bool3],..."
+ *
+ * The remaining part of "to_parse" is stored in "end" (if we are parsing
+ * the last candidate, then this is NULL) and the value (if given) is
+ * parsed and stored in "val", so "val" always points to either 0 or 1.
+ * If the value is not given, then "val" is set to point to 1.
+ *
+ * The boolean value is parsed using "git_parse_maybe_bool()", so the
+ * accepted values are
+ *
+ *	to set true  - "1", "yes", "true"
+ *	to set false - "0", "no", "false"
+ *
+ * This function returns 1 on success. It returns 0 when we don't find
+ * an exact match for "candidate" or when the boolean value given is
+ * not valid.
+ */
+static int match_atom_bool_arg(const char *to_parse, const char *candidate,
+				const char **end, int *val)
+{
+	const char *argval;
+	char *strval;
+	size_t arglen;
+	int v;
+
+	if (!match_atom_arg_value(to_parse, candidate, end, &argval, &arglen))
+		return 0;
+
+	if (!argval) {
+		*val = 1;
+		return 1;
+	}
+
+	strval = xstrndup(argval, arglen);
+	v = git_parse_maybe_bool(strval);
+	free(strval);
+
+	if (v == -1)
+		return 0;
+
+	*val = v;
+
+	return 1;
+}
+
 static int color_atom_parser(struct ref_format *format, struct used_atom *atom,
 			     const char *color_value, struct strbuf *err)
 {