Seems commit 6cecc26206c4a539999b04664136c6f785211a41 missed on dependancy, that is blaze-markup
Assistant/Threads/WebApp.hs:25:8:
    Could not find module `Text.Blaze.Renderer.String'
    It is a member of the hidden package `blaze-markup-0.5.1.0'.
    Perhaps you need to add `blaze-markup' to the build-depends in your .cabal file.
    Use -v to see a list of the files searched for.
cabal: Error: some packages failed to install:
git-annex-3.20120721 failed during the building phase. The exception was:
ExitFailure 1
This should fix it
x00:git-annex jtang$ git diff
diff --git a/git-annex.cabal b/git-annex.cabal
index c7d9bf5..4f98d2a 100644
--- a/git-annex.cabal
+++ b/git-annex.cabal
@@ -76,7 +76,7 @@ Executable git-annex
   if flag(Webapp)
     Build-Depends: yesod, yesod-static, case-insensitive, http-types,
      transformers, wai, wai-logger, warp, blaze-builder, blaze-html,
-     crypto-api, hamlet
+     blaze-markup, crypto-api, hamlet
     CPP-Options: -DWITH_WEBAPP
 
   if (os(darwin))
> [[done]].. interestingly, cabal had not complained about there here, > as in my version, it's in blaze, not blaze-markup. Added it anyway. > --[[Joey]]