Merge branch 'js/mingw-o-append'

Among the three codepaths we use O_APPEND to open a file for
appending, one used for writing GIT_TRACE output requires O_APPEND
implementation that behaves sensibly when multiple processes are
writing to the same file.  POSIX emulation used in the Windows port
has been updated to improve in this area.

* js/mingw-o-append:
  mingw: enable atomic O_APPEND