Removing fetchgit --depth and its usage. It does not provide much advantages...

I think it takes the recent N commits into the repository, which says very little,
even for wanting master/HEAD.

svn path=/nixpkgs/trunk/; revision=18277

pkgs/build-support/fetchgit/default.nix

@@ -1,5 +1,5 @@
 {stdenv, git}:
-{url, rev ? "HEAD", md5 ? "", sha256 ? "", depth ? 1}:
+{url, rev ? "HEAD", md5 ? "", sha256 ? ""}:
 
 stdenv.mkDerivation {
   name = "git-export";