nix-prefetch-git: Remove `--depth' argument since it's risky.

The `--depth' argument asks Git to fetch the last revisions of the given
repo on *any* branch, which is often useless.

Thanks to Lluís Battle for clarifying this.

svn path=/nixpkgs/trunk/; revision=18438

pkgs/build-support/fetchgit/nix-prefetch-git

@@ -36,7 +36,7 @@ if test -z "$finalPath"; then
     trap "rm -rf $tmpPath" EXIT
 
     # Perform the checkout.
-    git clone --depth 1 "$url" $tmpFile 1>&2
+    git clone "$url" $tmpFile 1>&2
     if test -n "$rev"; then
       cd $tmpFile
       echo $tmpFile