implement rcs_getmtime for svn

implement rcs_getmtime for svn

This is a slow implementation; it runs svn log once per file
still, rather than running svn log once on the whole srcdir.

I did it this way because in my experience, svn log, run on a directory,
does not always list every change to files inside that directory.
I don't know why, and I use svn as little as possible these days.