ref-filter: add deltabase option

Add new formatting option: deltabase.
If the object is stored as a delta on-disk, this expands
to the 40-hex sha1 of the delta base object.
Otherwise, expands to the null sha1 (40 zeroes).
We have same option in cat-file command.
Hopefully, in the end I will remove formatting code from
cat-file and reuse formatting parts from ref-filter.

Signed-off-by: Olga Telezhnaia <olyatelezhnaya@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

ref-filter.c

@@ -246,6 +246,18 @@ static int objectsize_atom_parser(const struct ref_format *format, struct used_a
 	return 0;
 }
 
+static int deltabase_atom_parser(const struct ref_format *format, struct used_atom *atom,
+				 const char *arg, struct strbuf *err)
+{
+	if (arg)
+		return strbuf_addf_ret(err, -1, _("%%(deltabase) does not take arguments"));
+	if (*atom->name == '*')
+		oi_deref.info.delta_base_sha1 = oi_deref.delta_base_oid.hash;
+	else
+		oi.info.delta_base_sha1 = oi.delta_base_oid.hash;
+	return 0;
+}
+
 static int body_atom_parser(const struct ref_format *format, struct used_atom *atom,
 			    const char *arg, struct strbuf *err)
 {
@@ -437,6 +449,7 @@ static struct {
 	{ "objecttype", SOURCE_OTHER, FIELD_STR, objecttype_atom_parser },
 	{ "objectsize", SOURCE_OTHER, FIELD_ULONG, objectsize_atom_parser },
 	{ "objectname", SOURCE_OTHER, FIELD_STR, objectname_atom_parser },
+	{ "deltabase", SOURCE_OTHER, FIELD_STR, deltabase_atom_parser },
 	{ "tree", SOURCE_OBJ },
 	{ "parent", SOURCE_OBJ },
 	{ "numparent", SOURCE_OBJ, FIELD_ULONG },
@@ -888,7 +901,8 @@ static void grab_common_values(struct atom_value *val, int deref, struct expand_
 		} else if (!strcmp(name, "objectsize")) {
 			v->value = oi->size;
 			v->s = xstrfmt("%lu", oi->size);
-		}
+		} else if (!strcmp(name, "deltabase"))
+			v->s = xstrdup(oid_to_hex(&oi->delta_base_oid));
 		else if (deref)
 			grab_objectname(name, &oi->oid, v, &used_atom[i]);
 	}