Add support for synchronizing ebuild and eclass timestamps with the metadata

Add support for synchronizing ebuild and eclass timestamps with the metadata
cache timestamps which emerge --sync operates on a git repository. This is
necessary since git doesn't preserve timestamps. It is assumed that the
timestamps should be synchronized if the relevant ebuilds and eclasses are
unmodified relative to the HEAD commit (as reported by git-ls-files). This
feature was requested by Daniel Robbins, so that it's possible for the funtoo
git repository to distribute pre-generated metadata.

svn path=/main/trunk/; revision=12303