There appears to be a bad ref on the server for the Cabal repo:

== running git submodule update

error: Unable to find b4d0c0f2846542dc2e2df189fe145a56ac9b30b6 under http://darcs.haskell.org/libraries/Cabal.git

Cannot obtain needed object b4d0c0f2846542dc2e2df189fe145a56ac9b30b6

while processing commit e7e7ce1029707a67d26e6dc29de11141734898e3.

error: Fetch failed.

Unable to fetch in submodule path 'libraries/Cabal'

git failed: 256 at ./sync-all line 116.