largefiles: ensure addlargefiles() doesn't add a standin as a largefile
An easy way to force this (and cause a traceback) prior to the fix for 3507 was
$ touch large
$ hg add --large large
$ hg ci -m "add"
$ hg remove large
$ touch large
$ hg addremove --config largefiles.patterns=**large
This patch also detected (and corrected) a previous test where a standin got
added as a largefile (without a traceback).