config: load included config files in binary mode
I guess we've been getting lucky that this works in Python 2. This
fixes loading included config files in Python 3 (it used to fail on
the "somebody set up us the BOM" check.)
$ hg init
$ echo This is file a1 > a
$ hg add a
$ hg commit -m "commit #0"
$ ls
a
$ echo This is file b1 > b
$ hg add b
$ hg commit -m "commit #1"
$ hg co 0
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
B should disappear
$ ls
a