py3: convert the mode argument of os.fdopen to unicodes (1 of 2)
os.fdopen() does not accepts bytes as its second argument which represent the
mode in which the file is to be opened. This patch makes sure unicodes are
passed in py3 by using pycompat.sysstr().
$ hg init
$ echo 123 > a
$ hg add a
$ hg commit -m "first" a
$ mkdir sub
$ echo 321 > sub/b
$ hg add sub/b
$ hg commit -m "second" sub/b
$ cat sub/b
321
$ hg co 0
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
$ cat sub/b 2>/dev/null || echo "sub/b not present"
sub/b not present
$ test -d sub || echo "sub not present"
sub not present