The pure version was mpatch was throwing struct.error or ValueError
for errors, whereas the C version was throwing an "mpatch.mpatchError".
Introducing an mpatch.mpatchError into pure and using it consistently
is fairly easy, but the actual form for it is mercurial.mpatch.mpatchError,
so with this commit, we change the C implementation to match the naming
convention too.
While I was here, I removed the try..except around importing cStringIO
because cStringIO should always be importable on modern Python versions.
We already do an unconditional import in other files.
In Python lists are implemented as arrays with overallocation. As a
result, list.insert(0, ...) is O(n), whereas list.append() has an
amortised running time of O(1). Reversing the internal representation
of the list should cause a slight speedup for pure Python builds.
