fetchgit: add 'deepClone' argument to disable shallow fetching

This patch resolves https://github.com/NixOS/nixpkgs/issues/6395. Deep
cloning is useful in combination with 'leaveDotGit' for builds that want
to run "git describe" to obtain a proper version string, etc., like the
'haskellngPackages.cabal2nix' package does.

pkgs/build-support/fetchgit/builder.sh

@@ -8,6 +8,7 @@ header "exporting $url (rev $rev) into $out"
 
 $fetcher --builder --url "$url" --out "$out" --rev "$rev" \
   ${leaveDotGit:+--leave-dotGit} \
+  ${deepClone:+--deepClone} \
   ${fetchSubmodules:+--fetch-submodules}
 
 stopNest