Merge branch 'jk/blame-commit-label' into maint

"git blame HEAD -- missing" failed to correctly say "HEAD" when it
tried to say "No such path 'missing' in HEAD".

* jk/blame-commit-label:
  blame.c: fix garbled error message
  use xstrdup_or_null to replace ternary conditionals
  builtin/commit.c: use xstrdup_or_null instead of envdup
  builtin/apply.c: use xstrdup_or_null instead of null_strdup
  git-compat-util: add xstrdup_or_null helper

walker.c

@@ -232,7 +232,7 @@ int walker_targets_stdin(char ***target, const char ***write_ref)
 			REALLOC_ARRAY(*write_ref, targets_alloc);
 		}
 		(*target)[targets] = xstrdup(tg_one);
-		(*write_ref)[targets] = rf_one ? xstrdup(rf_one) : NULL;
+		(*write_ref)[targets] = xstrdup_or_null(rf_one);
 		targets++;
 	}
 	strbuf_release(&buf);