Redefine `Merge::commit_merge()` to use `&mut self`. #237

Job	Run time
no_std	35s
build (ubuntu, stable, locked, true)	9m 8s
miri	1m 44s
build (windows, stable, locked, false)	32m 58s
build (macos, stable, locked, false)	19m 11s
build (ubuntu, nightly, locked, false)	18m 57s
build (ubuntu, beta, locked, false)	18m 30s
build (ubuntu, stable, latest, false)	10m 18s
build (ubuntu, stable, minimal, false)	10m 29s
fuzz	15m 19s
features	23m 14s
deploy	8s
	2h 40m 31s

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