The git checkout and build process can leave the checkout
date in the tarball release, leading to unstable sorting.
I tried to use `git restore-mtime`, but that doesn't work for
ikiwiki, because dgit interferes with it.
* 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.