Merge branch 'tb/ref-filter-empty-modifier' into maint

In the "--format=..." option of the "git for-each-ref" command (and
its friends, i.e. the listing mode of "git branch/tag"), "%(atom:)"
(e.g. "%(refname:)", "%(body:)" used to error out.  Instead, treat
them as if the colon and an empty string that follows it were not
there.

* tb/ref-filter-empty-modifier:
  ref-filter.c: pass empty-string as NULL to atom parsers

Author: gitster

ref-filter.c

@@ -415,8 +415,16 @@ static int parse_ref_filter_atom(const struct ref_format *format,
 	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)
+	if (arg) {
 		arg = used_atom[at].name + (arg - atom) + 1;
+		if (!*arg) {
+			/*
+			 * Treat empty sub-arguments list as NULL (i.e.,
+			 * "%(atom:)" is equivalent to "%(atom)").
+			 */
+			arg = NULL;
+		}
+	}
 	memset(&used_atom[at].u, 0, sizeof(used_atom[at].u));
 	if (valid_atom[i].parser)
 		valid_atom[i].parser(format, &used_atom[at], arg);

t/t6300-for-each-ref.sh

@@ -51,6 +51,7 @@ test_atom() {
 }
 
 test_atom head refname refs/heads/master
+test_atom head refname: refs/heads/master
 test_atom head refname:short master
 test_atom head refname:lstrip=1 heads/master
 test_atom head refname:lstrip=2 master