automatically run --gettime, and optimise it for git

automatically run --gettime, and optimise it for git

* Automatically run --gettime the first time ikiwiki is run on
  a given srcdir.
* Optimise --gettime for git, so it's appropriatly screamingly
  fast. (This could be done for other backends too.)
* However, --gettime for git no longer follows renames.
* Use above to fix up timestamps on docwiki, as well as ensure that
  timestamps on basewiki files shipped in the deb are sane.