Merge branch 'js/mingw-o-append' into maint

Further fix for O_APPEND emulation on Windows

* js/mingw-o-append:
mingw: fix mingw_open_append to work with named pipes
t0051: test GIT_TRACE to a windows named pipe

diff --cc Makefile
Simple merge

diff --cc t/helper/test-tool.c
Simple merge

diff --cc t/helper/test-tool.h
Simple merge