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dirstate: back out 502b56a9e897 Superseded by the parent of this commit.