rebase: clear merge state when aborting in-memory merge on dirty working copy

Differential Revision: https://phab.mercurial-scm.org/D9509

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branch : stable
36 jobs for 5.6.1 in 43 minutes and 42 seconds (queued for 7 minutes and 40 seconds)
latest
Status Job ID Name Coverage
  Test
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rust-cargo-test-py2

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rust-cargo-test-py3

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rust-cargo-test-py3-dirstate-tree

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test-py2

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test-py2-chg

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test-py2-pure

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test-py2-rust

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test-py3

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test-py3-pure

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test-py3-rust

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test-py3-rust-dirstate-tree

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checks-py2

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rust-cargo-test-py2

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rust-cargo-test-py3-dirstate-tree

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test-py2

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test-py2

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test-py2-chg

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test-py2-chg

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test-py2-rust

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test-py3

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test-py3-rust

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test-py3-rust-dirstate-tree

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test-py3-rust-dirstate-tree

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