We've noticed issues on our infrastucture iterating over the many
tag/branch/head reference files that some git repositories may contain.
By issuing the pack-refs command, we move these all to a single file
which speeds up operations with the mirror repos in the downloads
directory in general.
(Bitbake rev: f8126aaf774186a6eaf0bd4067b89c074594886c)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>