Instead of duplicating the permissions on the temp file, the permissions
are duplicated on the output file directly. If the output file exists as
read-only, it is made writeable first.
There are some environments which don't allow the mv to work if the file
itself is read-only.
A new variable is exposed to templates, which holds the filename of the
template source. The primary use case is to be able to include a warning
message within the template. For example:
# Do not edit. This file auto-generated from {{ yadm.source }}.
