avoid quadratic per-leaf copy in StringMerger.do_transform

[sahvx655-wq]

Description

Profiling --preview string merging on "%s ..." % (a, b, c, ...) still showed a quadratic after #5199 linearised append_leaves: the run sat around 4s for 8000 operands with len called about 32 million times. StringMerger.do_transform rebuilds the merged line by walking every leaf of the old line and copying each leaf that is not part of a merged string with its own append_leaves(new_line, line, [leaf]) call. The resume hint added in #5199 only persists inside a single append_leaves call, so a one-leaf call always restarts the search for the leaf's position from the front. A % expression's operand tuple is a single parent holding every operand, so copying its n leaves one at a time rescans that child list from index 0 each time and is O(n²). It is reachable pre-auth from blackd with X-Preview and X-Enable-Unstable-Feature: string_processing.

I gather the consecutive leaves that fall outside any merged string group and hand each contiguous run to append_leaves in one call, which is the shape the resume hint was built for, so the copy stays linear. The per-leaf calls were doing nothing the batched call does not, comments included, so the new line is built identically. Output is byte-identical across the test suite and the black source tree in stable, preview and unstable modes plus crafted %-format and implicit-concat inputs; the 8000-operand case drops from roughly 4.1s to 0.8s and the curve flattens from quadratic to linear.

Checklist - did you ...

• Implement any code style changes under the --preview style, following the stability policy?
• Add an entry in CHANGES.md if necessary?
• Add / update tests if necessary?
• Add new / update outdated documentation?

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diff-shades results comparing this PR (719b3ee) to main (d7587ce):

--preview style: no changes

--stable style: no changes

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