scmutil: don't append .orig to backups in origbackuppath (BC)
When ui.origbackuppath is set, .orig files are stored outside of the working
copy, however they still have a .orig suffix appended to them. This can cause
unexpected conflicts, particularly when tracked files or directories have .orig
at the end.
This change removes the .orig suffix from files stored in an out-of-tree
origbackuppath.
Test Plan:
Update and run unit tests.
Differential Revision: https://phab.mercurial-scm.org/D679