convert: test clean p2 file missing 216fa1ba9993 introduced "clever" reuse of p2 but did that convert could fail with abort: f1@f73e02ae52c5: not found in manifest! when it tried to reuse a file from p2 but the file didn't exist there, for example because filemap changes. 5ca587348875 fixed that (using changes from a75d24539aba), but with a quite different reasoning and test case. Add another test that makes sure this case is covered too.