subrepo: make update -C clean the working directory for svn subrepos
This makes 'hg update --clean' behave the same way for both kinds of
subrepositories. Before Subversion subrepos did not take the clean
parameter into account, but just updated to the given revision and
merged uncommitted changes into that.
$ hg init
$ echo 0 > a
$ echo 0 > b
$ hg ci -A -m m
adding a
adding b
$ hg rm a
$ hg cat a
0
$ hg cat --decode a # more tests in test-encode
0
$ echo 1 > b
$ hg ci -m m
$ echo 2 > b
$ hg cat -r 0 a
0
$ hg cat -r 0 b
0
$ hg cat -r 1 a
a: no such file in rev 7040230c159c
[1]
$ hg cat -r 1 b
1