largefiles: test that a largefile cannot accidentally be re-added as a regular file

backout dbdb777502dc (issue3077) (issue3071) Using util.realpath turns out to create complex issues on both Mac and Windows. Back this change out for the release.

largefiles: fix uppercase in abort message

Makefile: adjust example, test-merge1 is now test-merge1.t

tests: use $DAEMON_PIDS and killdaemons in largefiles test $DAEMON_PIDS is used by tests to make sure there will be no leftover processes, the cycling through ports is needed because they are not available for a new bind that quickly on some systems. There are not enough $HGPORT variables available for each hg serve, so use the killdaemons script before reusing ports.

tests: make largefiles test use $HGPORT instead of 8001

merge in another fix on the wrong branch