rust: fix warning about unnecessary mut
If there's a reason to use mut (like compability with older
compilers), then we should stick `#[allow(unused_mut)]` on the
declaration.
Differential Revision: https://phab.mercurial-scm.org/D8538
#require serve
$ hg clone http://localhost:$HGPORT/ copy
abort: * (glob)
[255]
$ test -d copy
[1]
$ "$PYTHON" "$TESTDIR/dumbhttp.py" -p $HGPORT --pid dumb.pid
$ cat dumb.pid >> $DAEMON_PIDS
$ hg clone http://localhost:$HGPORT/foo copy2
abort: HTTP Error 404: * (glob)
[255]
$ killdaemons.py