mergestate: fix BC breakage introduced because of removal of a merge record

In fcd0cff3400a I removed the usage of RECORD_RESOLVED_OTHER. However I also
removed the reading support, hence if there is any user who runs into merge
conflict with older version of hg, upgrades hg and then tries to read
mergestate, they will end up with an MergeRecordUnsupportedError.

--HG--
branch : stable
11 jobs for 5.5 in 29 minutes and 30 seconds (queued for 19 minutes and 54 seconds)
latest
Status Job ID Name Coverage
  Test
passed #89947
checks-py2

00:02:43

passed #89948
checks-py3

00:04:07

passed #89949
rust-cargo-test-py2

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passed #89950
rust-cargo-test-py3

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passed #89951
test-py2

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passed #89957
test-py2-chg

00:22:35

passed #89953
test-py2-pure

00:20:42

passed #89955
test-py2-rust

00:23:33

passed #89952
test-py3

00:12:40

passed #89954
test-py3-pure

00:19:08

passed #89956
test-py3-rust

00:18:55