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