Do not clobber the previously written content of a log file.
Do not clobber the previously written content of a log file.

In the "patch" task execute() is called many times, so log should be
written in append mode.

Based on the work of Akinori MUSHA<knu@idaemons.org> in
    https://github.com/flavorjones/mini_portile/pull/34

flavorjones/mini_portile,luislavena/mini_portile,flavorjones/mini_portile,jtarchie/mini_portile,luislavena/mini_portile