state: created new class statecheck to handle unfinishedstates For the purpose of handling states for various multistep operations like `hg graft`, `hg histedit`, `hg bisect` et al a new class called statecheck is created .This will help in having a unified approach towards these commands and handle them with ease. The class takes in 4 basic arguments which include the name of the command, the name of the state file associated with it , clearable flag , allowcommit flag. This also also adds the support of`checkunfinished()` and `clearunfinished()` to the class. Tests remain unchanged. Differential Revision: https://phab.mercurial-scm.org/D6501

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