Caused by rBf7386b97571e.
Logic in BKE_bpath_traverse_main calls the callback multiple times [as
often as there are images in the strip].
Prior to above commit, the callback was
'bpath_relative_convert_visit_cb' [this one did not have this problem -
since it returned early if the path was already made relative once]
After rBf7386b97571e though, the 'bpath_relative_rebase_visit_cb' is
used [this one should not be entered multiple times, it would modifiy the
directy again and again].
Luckily, we have a flag (BKE_BPATH_TRAVERSE_SKIP_MULTIFILE) that can be
used to prevent this (this will take care of only calling the callback
once in BKE_bpath_traverse_main for the VSE case)