builtin/refs: new command to migrate ref storage formats #37

Job	Run time
config	2s
fuzz smoke test	44s
win+VS build	0s
documentation	2m 51s
sparse	2m 50s
linux32 (daald/ubuntu32:xenial)	2m 35s
linux-musl (alpine)	2m 38s
pedantic (fedora)	1m 45s
static-analysis	2m 49s
win+VS test	0s
win build	2m 41s
linux-asan-ubsan (ubuntu-latest)	2m 29s
linux-gcc (ubuntu-20.04)	2m 51s
linux-gcc-default (ubuntu-latest)	2m 50s
linux-leaks (ubuntu-latest)	2m 50s
linux-reftable (ubuntu-latest)	2m 50s
linux-reftable-leaks (ubuntu-latest)	2m 50s
linux-sha256 (ubuntu-latest)	2m 39s
linux-TEST-vars (ubuntu-20.04)	2m 31s
osx-clang (macos-13)	2m 29s
osx-gcc (macos-13)	2m 32s
osx-reftable (macos-13)	1m 40s
win test	0s
	47m 26s

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