mergestate: add a constructor that reads state from disk At the moment it's the same as just creating a new mergestate, but we'll soon move the _read call out of __init__ and in here.

merge.applyupdates: switch to mergestate.clean() See the previous patches for why we're doing this.

localrepo: switch to mergestate.clean() See the previous patches for why we're doing this.

strip: switch to mergestate.clean() See the previous patches for why we're doing this.

mergestate: add a constructor that sets up a clean merge state Eventually, we'll move the read call out of the constructor. This will: - avoid unnecessary reads when we're going to nuke the merge state anyway - avoid raising an exception if there's an unsupported merge record 'clean' seems like a good name for it because I wanted to avoid anything with the word 'new' in it, and 'reset' is more an action performed on a merge state than a way to get a new merge state. Thanks to Martin von Zweigbergk for feedback about naming this.

mergestate: raise structured exception for unsupported merge records We're going to catch this exception in 'hg summary' to print a better error message. This code is pretty untested, so there are no changes to test output. In upcoming patches we're going to test the output more thoroughly.