Let the update_diff fail if we are not in git repo

rc/extra/git-tools.kak

@@ -97,7 +97,6 @@ Available commands:\n  add\n  rm\n  blame\n  commit\n  checkout\n  diff\n  hide-
     }
 
     update_diff() {
-        git rev-parse --show-toplevel >/dev/null 2>&1 || return
         git --no-pager diff -U0 "$kak_buffile" | perl -e '
             $flags = $ENV{"kak_timestamp"};
             foreach $line (<STDIN>) {