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.

error: add a structured exception for unsupported merge records We're going to use this in summary to print a better error message.

dirstate: back out 502b56a9e897 Superseded by the parent of this commit.

osutil: make statfiles check for interrupts periodically This is a simpler and faster fix for issue4878 than the contortions performed in 502b56a9e897.

osutil: don't leak on statfiles error Found using the power of the mighty eyeball.