rebase: fix rebase aborts when 'tip-1' is public (issue4082) When aborting a rebase where tip-1 is public, rebase would fail to undo the merge state. This caused unexpected dirstate parents and also caused unshelve to become unabortable (since it uses rebase under the hood). The problem was that rebase uses -2 as a marker rev, and when it checked for immutableness during the abort, -2 got resolved to the second to last entry in the phase cache. Adds a test for the fix. Add exception to phase code to prevent this in the future.