ref-filter: introduce parsing functions for each valid atom

Parsing atoms is done in populate_value(), this is repetitive and
hence expensive. Introduce a parsing function which would let us parse
atoms beforehand and store the required details into the 'used_atom'
structure for further usage.

Helped-by: Eric Sunshine <sunshine@sunshineco.com>
Helped-by: Ramsay Jones <ramsay@ramsayjones.plus.com>
Helped-by: Andreas Schwab <schwab@linux-m68k.org>
Signed-off-by: Karthik Nayak <Karthik.188@gmail.com>
Reviewed-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

ref-filter.c

@@ -36,6 +36,7 @@ static int need_color_reset_at_eol;
 static struct {
 	const char *name;
 	cmp_type cmp_type;
+	void (*parser)(struct used_atom *atom, const char *arg);
 } valid_atom[] = {
 	{ "refname" },
 	{ "objecttype" },
@@ -114,6 +115,7 @@ struct atom_value {
 int parse_ref_filter_atom(const char *atom, const char *ep)
 {
 	const char *sp;
+	const char *arg;
 	int i, at;
 
 	sp = atom;
@@ -132,16 +134,16 @@ int parse_ref_filter_atom(const char *atom, const char *ep)
 	/* Is the atom a valid one? */
 	for (i = 0; i < ARRAY_SIZE(valid_atom); i++) {
 		int len = strlen(valid_atom[i].name);
+
 		/*
 		 * If the atom name has a colon, strip it and everything after
 		 * it off - it specifies the format for this entry, and
 		 * shouldn't be used for checking against the valid_atom
 		 * table.
 		 */
-		const char *formatp = strchr(sp, ':');
-		if (!formatp || ep < formatp)
-			formatp = ep;
-		if (len == formatp - sp && !memcmp(valid_atom[i].name, sp, len))
+		arg = memchr(sp, ':', ep - sp);
+		if (len == (arg ? arg : ep) - sp &&
+		    !memcmp(valid_atom[i].name, sp, len))
 			break;
 	}
 
@@ -154,6 +156,10 @@ int parse_ref_filter_atom(const char *atom, const char *ep)
 	REALLOC_ARRAY(used_atom, used_atom_cnt);
 	used_atom[at].name = xmemdupz(atom, ep - atom);
 	used_atom[at].type = valid_atom[i].cmp_type;
+	if (arg)
+		arg = used_atom[at].name + (arg - atom) + 1;
+	if (valid_atom[i].parser)
+		valid_atom[i].parser(&used_atom[at], arg);
 	if (*atom == '*')
 		need_tagged = 1;
 	if (!strcmp(used_atom[at].name, "symref"))