better handle single-character expansions. Fixes #2216 #8698

Job	Run time
DEB build (ubuntu-20.04, ubuntu:20.04)	2m 25s
DEB build (ubuntu-22.04, ubuntu:22.04)	2m 47s
DEB build (ubuntu-24.04, ubuntu:24.04)	4m 1s
DEB build (debian-10, debian:buster)	2m 21s
DEB build (debian-11, debian:bullseye)	2m 19s
DEB build (debian-12, debian:bookworm)	2m 57s
DEB build (debian-sid, debian:sid)	3m 13s
DEB install test (ubuntu-20.04, ubuntu:20.04)	30s
DEB install test (ubuntu-22.04, ubuntu:22.04)	21s
DEB install test (ubuntu-24.04, ubuntu:24.04)	23s
DEB install test (debian-10, debian:buster)	27s
DEB install test (debian-11, debian:bullseye)	21s
DEB install test (debian-12, debian:bookworm)	20s
DEB install test (debian-sid, debian:sid)	26s
	22m 51s

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