dirstate-v2: Add a zero-size error type for dirstate v2 parse errors
This error should only happen if Mercurial is buggy or the file is corrupted.
It indicates for example that:
* A part of the file refers to another part, and the byte offset or item count
would cause reading out of bounds, beyond the end of the file.
* The byte for an entry state has an invalid value
When parsing becomes lazy, many more functions will return a `Result` with
this error. Making it zero-size reduces the work that the `?` operator needs
to do to pass around the error value.
Differential Revision: https://phab.mercurial-scm.org/D10748
local test passed
addgroupcopy test passed
clone test passed
lowlevelcopy test passed
slicing test passed
issnapshot test passed
findsnapshot test passed