ref-filter: resolve HEAD when parsing %(HEAD) atom

If the user asks to display (or sort by) the %(HEAD) atom,
ref-filter has to compare each refname to the value of HEAD.
We do so by resolving HEAD fresh when calling populate_value()
on each ref. If there are a large number of refs, this can
have a measurable impact on runtime.

Instead, let's resolve HEAD once when we realize we need the
%(HEAD) atom, allowing us to do a simple string comparison
for each ref. On a repository with 3000 branches (high, but
an actual example found in the wild) this drops the
best-of-five time to run "git branch >/dev/null" from 59ms
to 48ms (~20% savings).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

ref-filter.c

@@ -92,6 +92,7 @@ static struct used_atom {
 			unsigned int length;
 		} objectname;
 		struct refname_atom refname;
+		char *head;
 	} u;
 } *used_atom;
 static int used_atom_cnt, need_tagged, need_symref;
@@ -286,6 +287,12 @@ static void if_atom_parser(struct used_atom *atom, const char *arg)
 	}
 }
 
+static void head_atom_parser(struct used_atom *atom, const char *arg)
+{
+	unsigned char unused[GIT_SHA1_RAWSZ];
+
+	atom->u.head = resolve_refdup("HEAD", RESOLVE_REF_READING, unused, NULL);
+}
 
 static struct {
 	const char *name;
@@ -324,7 +331,7 @@ static struct {
 	{ "push", FIELD_STR, remote_ref_atom_parser },
 	{ "symref", FIELD_STR, refname_atom_parser },
 	{ "flag" },
-	{ "HEAD" },
+	{ "HEAD", FIELD_STR, head_atom_parser },
 	{ "color", FIELD_STR, color_atom_parser },
 	{ "align", FIELD_STR, align_atom_parser },
 	{ "end" },
@@ -1366,12 +1373,7 @@ static void populate_value(struct ref_array_item *ref)
 		} else if (!deref && grab_objectname(name, ref->objectname, v, atom)) {
 			continue;
 		} else if (!strcmp(name, "HEAD")) {
-			const char *head;
-			unsigned char sha1[20];
-
-			head = resolve_ref_unsafe("HEAD", RESOLVE_REF_READING,
-						  sha1, NULL);
-			if (head && !strcmp(ref->refname, head))
+			if (atom->u.head && !strcmp(ref->refname, atom->u.head))
 				v->s = "*";
 			else
 				v->s = " ";