Add a new egencache --rsync option which enables a stat collision workaround

Add a new egencache --rsync option which enables a stat collision workaround
for cases in which the content of a cache entry changes and neither the file
mtime nor size changes (preventing rsync from detecting changes). See bug
#139134. This option should only be needed for distribution via something
like rsync, which relies on timestamps and file sizes to detect changes. It's
not needed with git since that uses a more thorough mechanism which allows it
to detect changed inode numbers (described in racy-git.txt in the git
technical docs).

svn path=/main/trunk/; revision=13262