ui: hide fin/fout/ferr attributes behind @property functions
This allows keeping references to fout/ferr/fin which are updated when these
properties are changed. See the next patch.
Set up
$ hg init repo
$ cd repo
Try to import an empty patch
$ hg import --no-commit - <<EOF
> EOF
applying patch from stdin
abort: stdin: no diffs found
[255]
No dirstate backups are left behind
$ ls .hg/dirstate* | sort
.hg/dirstate