At least some packages within the sstate tree (probably
only -native ones), contain references to the working tree in which
they were built, hence making them unsuitable for sharing.

Here's how to see this problem:
* step 1 - build Poky baseline
  % . /local/poky-base/oe-init-build-env /local/poky_build1
  % bitbake git-native

* step 2 - rebuild, using baseline for SSTATE_MIRRORS
  % . /local/poky-base/oe-init-build-env /local/poky_build2
  -- adjust conf/local.conf to contain these lines:
    # Reusable state information
    SSTATE_MIRRORS ?= "\
    file://.* file:///local/poky_build1/sstate-cache/"
  % bitbake git-native

In this case, 'git-native' only comes from the SSTATE packaging
and will just be installed into tmp/sysroots.  
  % /local/poky_build2/tmp/sysroots/i686-linux/usr/bin/git repack
  fatal: Not a git repository (or any of the parent directories): .git
so it works fine.  Now, to break it:
  % mv /local/poky_build1/tmp{,.ORIG}
  % /local/poky_build2/tmp/sysroots/i686-linux/usr/bin/git repack
  git: 'repack' is not a git command. See 'git --help'.

Note: I'm quite sure this problem is not isolated to 'git-native'