Introduce an on-disk file to record revindex for packdata, which
traditionally was always created on the fly and only in-core.
* tb/pack-revindex-on-disk:
t5325: check both on-disk and in-memory reverse index
pack-revindex: ensure that on-disk reverse indexes are given precedence
t: support GIT_TEST_WRITE_REV_INDEX
t: prepare for GIT_TEST_WRITE_REV_INDEX
Documentation/config/pack.txt: advertise 'pack.writeReverseIndex'
builtin/pack-objects.c: respect 'pack.writeReverseIndex'
builtin/index-pack.c: write reverse indexes
builtin/index-pack.c: allow stripping arbitrary extensions
pack-write.c: prepare to write 'pack-*.rev' files
packfile: prepare for the existence of '*.rev' files
3c12d0b885