fileset: switch to mergestate.read() See previous patches for why we're doing this.

commands.summary: switch to mergestate.read() See previous patch for why we're doing this. We do this with a bit of care -- it would be bad form for 'hg summary' to abort completely if we encounter an unsupported merge record. Instead just warn about that and continue with the rest of the summary.

commands.resolve: switch to mergestate.read() See previous patches for why we're doing this.

shelve: switch to mergestate.read() See previous patches for why we're doing this.

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.