nix-prefetch-git: Fix inconsistency with fetchgit regarding deepClone

The fetchgit function in nixpkgs sets the leaveDotGit argument to true
if deepClone is set to true. nix-prefetch-git did behave differently. It
would not assume --leave-dotGit if --deepClone is specified. With this
change the inconsistency is addressed by assuming --leave-dotGit if
--deepClone is specified.

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

@@ -105,6 +105,10 @@ for arg; do
     fi
 done
 
+if test -n $deepClone; then
+    leaveDotGit=true
+fi
+
 if test -z "$url"; then
     usage
 fi