AmbiguousCommand is raised too soon. Right now, hg raises AmbiguousCommand as soon as it finds two commands/aliases that start with the substring it's searching for, even though it may still find a full match later on. This is a bit hard to hit on purpose, because hg checks the list of commands in whatever order is returned by table.keys(), which will change when you add an alias to a command. You should be able to hit it by adding an alias "u" to the "identify" command - not that that makes a lot of sense...

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requesting all changes

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adding changesets

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adding manifests

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adding file changes

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added 1 changesets with 1 changes to 1 files

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checking changesets

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checking manifests

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crosschecking files in changesets and manifests

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checking files

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1 files, 1 changesets, 1 total revisions