[perf] Skip the newline count on chunks without a newline

In ``pformat_lines``'s budget loop, ``chunk.count("\n")`` ran on every
chunk, but most chunks (brackets, indentation, item reprs) contain no
newline. Guarding the call with ``"\n" in chunk`` skips it on those and
recovers part of the per-chunk budget-tracking overhead: formatting an
8-element list under a budget drops from ~0.0185 ms to ~0.0163 ms
(versus ~0.0132 ms for an uncapped ``pformat().splitlines()``, so the
budget overhead roughly halves, from ~+5 us to ~+3 us).

The win is small and only matters on the ``-v`` truncating path of a
failing assertion (the default path doesn't format the diff at all), so
this is kept as a separate commit — easy to drop if the extra branch
isn't judged worth it.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: Pierre-Sassoulas

src/_pytest/_io/pprint.py

@@ -124,7 +124,10 @@ def pformat_lines(
             chunks.append(chunk)
             if max_chars is not None:
                 n_chars += len(chunk)
-            if max_lines is not None:
+            if max_lines is not None and "\n" in chunk:
+                # Guard the count: most chunks (brackets, indents, item
+                # reprs) have no newline, and skipping the call on them
+                # is meaningfully cheaper than counting every chunk.
                 n_lines += chunk.count("\n")
             if (max_lines is not None and n_lines >= max_lines) or (
                 max_chars is not None and n_chars >= max_chars