libsubprocess: do not spin on large lines #9580

Job	Run time
python linting	30s
validate commits	13s
Generate build matrix	6s
spelling	4s
bookworm - test-install	12m 9s
bookworm - 32 bit	13m 47s
noble - test-install	12m 44s
el9 - test-install	12m 46s
alpine - test-install	12m 24s
fedora40 - test-install	12m 36s
jammy - test-install	11m 41s
focal - py3.8	16m 59s
bookworm - gcc-12,content-s3,distcheck	20m 15s
fedora40 - clang-18	17m 31s
coverage	24m 26s
el8 - ascii	19m 37s
el8 - system,coverage	7m 52s
inception	22m 54s
flux-pam check	3m 52s
flux-sched check	6m 41s
flux-pmix check	13m 12s
flux-accounting check	4m 45s
Generate docker manifest	1s
	4h 7m 5s

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