fetchgit: remove only .git

Source of this change goes back to 2009 and original version of
fetchgit at 205fb0c87e.

The nondeterminism is really caused by changing .git so leave other
files alone as they might be interesting.

Note: this causes a hash mismatch with Hydra's version of Git Plugin
which we should fix to comply.

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

@@ -286,7 +286,7 @@ _clone_user_rev() {
     eval "$NIX_PREFETCH_GIT_CHECKOUT_HOOK"
     if test -z "$leaveDotGit"; then
         echo "removing \`.git'..." >&2
-        find "$dir" -name .git\* -print0 | xargs -0 rm -rf
+        find "$dir" -name .git -print0 | xargs -0 rm -rf
     else
         find "$dir" -name .git | while read gitdir; do
             make_deterministic_repo "$(readlink -f "$gitdir/..")"