Don't apply self-referential replacements twice

[gaoflow]

The problem

Custom replacements are applied in two passes: once before unidecode (line ~109) and once in the finalize step (line ~186). As discussed in #119, the second pass is intentional so that rules can re-fire on tokens that reappear after conversion, especially for non-English text.

That second pass corrupts a self-referential rule, where the replacement value contains its own search key. The finalize pass re-fires on the first pass's output and the slug grows on every run:

slugify('a', replacements=[['a', 'aa']])        # -> 'aaaa'   (expected 'aa')
slugify('hello', replacements=[['l', 'll']])     # -> 'hellllllllo'  (expected 'hellllo')

The fix

Keep the intentional finalize re-scan, but skip the rules where re-application is provably self-corrupting (old in new) — those were already applied in the first pass and cannot be safely re-applied:

for old, new in replacements:
    if old and old in new:
        continue
    text = text.replace(old, new)

Every non-self-referential rule still gets the second pass exactly as before, so the re-scan behavior from #119 is untouched. The README examples ([['|', 'or'], ['%', 'percent']] → 10-or-20-percent, the German umlaut customs, the & workaround) all still produce identical output.

Tests

Added test_replacements_not_applied_twice to TestSlugify; it fails before the change ('aaaa' != 'aa') and passes after. Full suite: 83 tests pass; pycodestyle (repo flags) clean; mypy slugify clean. Added a CHANGELOG entry under Unreleased.

Since the two-pass behavior is deliberate, I kept the change as narrow as possible — happy to adjust if you'd prefer a different approach (e.g. an explicit pre/post split).

I used an AI assistant to help investigate this under my direction, and I have reviewed and verified the change myself.