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.
* Rename --getctime to --gettime. (The old name still works for
backwards compatability.)
* --gettime now also looks up last modification time.
* Add rcs_getmtime to plugin API; currently only implemented
for git.