Takes the following arguments:
Reference to a hash, where each key is a directory to copy from and the
value is the directory to copy into. The whole tree below
the "from" directory is copied, with timestamps and permissions preserved.
The hash has two additional keys:
. After copying everything,
writes the list of
target files to the file named by
. If there is
another file named by
, its contents
are merged into the written file.