Merge branch 'ep/maint-equals-null-cocci'

Introduce and apply coccinelle rule to discourage an explicit
comparison between a pointer and NULL, and applies the clean-up to
the maintenance track.

* ep/maint-equals-null-cocci:
  tree-wide: apply equals-null.cocci
  tree-wide: apply equals-null.cocci
  contrib/coccinnelle: add equals-null.cocci

builtin/checkout.c

@@ -834,7 +834,7 @@ static int merge_working_tree(const struct checkout_opts *opts,
 			if (ret)
 				return ret;
 			o.ancestor = old_branch_info->name;
-			if (old_branch_info->name == NULL) {
+			if (!old_branch_info->name) {
 				strbuf_add_unique_abbrev(&old_commit_shortname,
 							 &old_branch_info->commit->object.oid,
 							 DEFAULT_ABBREV);