automata: fix bug where reverse NFA lacked an unanchored prefix #2158

Job	Run time
msrv	3m 35s
rustfmt	11s
miri-regex-automata	7m 14s
test (stable, ubuntu-latest, stable)	3m 55s
test (stable-32, ubuntu-latest, stable, i686-unknown-linux-gnu)	2m 41s
testfull-regex	4m 3s
testfull-regex-automata	8m 37s
test (stable-powerpc64, ubuntu-latest, stable, powerpc64-unknown-linux-gnu)	7m 44s
testfull-regex-capi	53s
test (stable-s390x, ubuntu-latest, stable, s390x-unknown-linux-gnu)	9m 18s
testfull-regex-syntax	2m 10s
test (beta, ubuntu-latest, beta)	3m 52s
test (nightly, ubuntu-latest, nightly)	3m 54s
test (macos, macos-latest, stable)	5m 0s
test (win-msvc, windows-latest, stable)	6m 16s
test (win-gnu, windows-latest, stable-x86_64-gnu)	3m 34s
	1h 12m 57s

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