check for source and target directory is the same

[jtrmal]

@danpovey this is not what we usually do, but I believe the change makes sense to simplify the workflow...
the typical use case the benefit becomes more visible is explicit UNK model:

  utils/lang/make_unk_lm.sh data/local/dict exp/make_unk
  utils/prepare_lang.sh \
    --unk-fst exp/make_unk/unk_fst.txt --phone-symbol-table data/lang/phones.txt \
    data/local/dict "<oov>" data/local/lang_test data/lang_test
  utils/format_lm.sh \
    data/lang_test data/srilm/best_3gram.gz data/local/dict/lexicon.txt data/lang_test

without this change, we'd have to create one intermediary lang directory to stand between prepare_lang and format_lm -- no big deal but maybe this is slightly nicer.

[danpovey]

I think people may not know what -ef is, might be nice to explain it.

[jtrmal]

ok, added explanation (please squash during merge)