range-diff: optionally include merge commits' diffs in the analysis #1863

Job	Run time
config	6s
fuzz smoke test	35s
win+VS build	0s
win build	5m 13s
sparse	5m 20s
linux32 (i386/ubuntu:focal)	5m 8s
linux-musl (alpine)	5m 10s
pedantic (fedora)	1m 42s
static-analysis	3m 41s
documentation	3m 22s
win+VS test	0s
linux-asan-ubsan (ubuntu-latest)	5m 19s
linux-gcc (ubuntu-20.04)	5m 20s
linux-gcc-default (ubuntu-latest)	5m 25s
linux-leaks (ubuntu-latest)	5m 20s
linux-reftable (ubuntu-latest)	5m 19s
linux-reftable-leaks (ubuntu-latest)	5m 18s
linux-sha256 (ubuntu-latest)	5m 18s
linux-TEST-vars (ubuntu-20.04)	5m 18s
osx-clang (macos-13)	5m 34s
osx-gcc (macos-13)	5m 37s
osx-reftable (macos-13)	5m 34s
win test	1s
	1h 29m 40s

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