Fix diff-hl-fallback-to-margin breakage after revert-buffer-quick #42

Job	Run time
check (26.1, false)	24s
check (26.3, false)	22s
check (27.2, false)	24s
check (release-snapshot, false)	23s
check (snapshot, true)	20s
check (28.2, false)	22s
check (29.4, false)	22s
check (30.1, false)	24s
	3m 1s

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