Add back fast path for non-gappy syncs #773

Job	Run time
changes	2s
check-lockfile	9s
lint-newsfile	33s
lint-crlf	4s
lint	43s
check-sampleconfig	42s
check-schema-delta	39s
Typechecking	1m 38s
lint-pydantic	55s
lint-clippy	0s
lint-clippy-nightly	0s
lint-rustfmt	0s
linting-done	3s
calculate-test-jobs	4s
trial-olddeps	7m 1s
portdb (3.8, 11)	2m 50s
complement (monolith, SQLite)	12m 6s
portdb (3.11, 15)	2m 36s
trial-pypy	0s
complement (monolith, Postgres)	13m 2s
complement (workers, Postgres)	21m 13s
cargo-test	0s
cargo-bench	0s
trial (3.8, sqlite, all)	4m 19s
sytest (focal)	8m 43s
trial (3.8, postgres, 11, all)	10m 6s
sytest (focal, postgres)	10m 58s
trial (3.8, sqlite)	4m 19s
sytest (focal, multi-postgres, workers)	13m 46s
sytest (focal, multi-postgres, workers, asyncio)	19m 40s
export-data	2m 3s
tests-done	2s
	2h 18m 16s

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