sync: fix deadlock with --track-renames-strategy modtime - fixes #4427
sync: fix deadlock with --track-renames-strategy modtime - fixes #4427

Before this change we could exit the popRenameMap function with the
lock held.

This fixes the problem by defer-ring the unlock.

See: https://forum.rclone.org/t/track-renames-strategy-modtime-doesnt-work/16992

ncw/rclone,ncw/rclone,ncw/rclone,ncw/rclone