don't call "git archive" with empty pathspec

When called from the top of the repository, $git_prefix is empty, and
setting checkoutroot to $git_prefix implies calling git with an empty
string as pathspec. This behavior was accepted as meaning "all path" in
old Git, but now triggers a warning and will soon be completely
forbidden.

Fixes #28.

git-latexdiff

@@ -535,7 +535,7 @@ if test "$ln_untracked" = 1; then
     ) || true
 fi
 
-if test "$subtree" = 1; then
+if test "$subtree" = 1 && test -n "$git_prefix"; then
     checkoutroot=$git_prefix
 else
     checkoutroot="."