Merge branch 'dev/merge_files' into 'master'

fix for race condition in merge_files.py

semi-regularly, Atlas has build failures due to race conditions from merge_files.py:

https://its.cern.ch/jira/browse/ATEAM-151

https://its.cern.ch/jira/browse/GAUDI-1079

here is the fix proposed by Scott and implemented by Rolf.

See merge request !21