rust-revlog: fix incorrect results with NULL_NODE prefixes In case a short hash is a prefix of `NULL_NODE`, the correct revision number lookup is `NULL_REVISION` only if there is no match in the nodemap. Indeed, if there is a single nodemap match, then it is an ambiguity with the always matching `NULL_NODE`. Before this change, using the Mercurial development repository as a testbed (it has public changesets with node ID starting with `0005` and `0009`), this is what `rhg` did (plain `hg` provided for reference) ``` $ rust/target/debug/rhg cat -r 000 README README: no such file in rev 000000000000 $ hg cat -r 000 README abort: ambiguous revision identifier: 000 ``` Here is the expected output for `rhg` on ambiguous prefixes (again, before this change): ``` $ rust/target/debug/rhg cat -r 0001 README abort: ambiguous revision identifier: 0001 ``` The test provided by 8c29af0f6d6e in `test-rhg.t` could become flaky with this change, unless all hashes are fixed. We expect reviewers to be more sure about that than we are.