bdiff: one more safe call of bdiff_freehunks(NULL)

bdiff: document that bdiff_freehunks() accepts NULL blocks() of cext/bdiff.c may pass NULL on OOM.

hgweb: propagate http headers from ErrorResponse for web interface commands This makes it possible for e.g. authorization hooks to provide appropriate headers to make the web browser ask for credentials. It's done in the same way as the existing code in wireprotoserver.py.