svnsubrepo: add new method _svnmissing
This is modelled after _gitmissing from dea6efdd7ec4 and also
necessary for svn. I'm not sure exactly how this hasn't been more of a
problem for svn until now, actually.
https://bz.mercurial-scm.org/1089
$ hg init
$ mkdir a
$ echo a > a/b
$ hg ci -Am m
adding a/b
$ hg rm a
removing a/b
$ hg ci -m m a
$ mkdir a b
$ echo a > a/b
$ hg ci -Am m
adding a/b
$ hg rm a
removing a/b
$ cd b
Relative delete:
$ hg ci -m m ../a
$ cd ..