merge stable heads

Added signature for changeset 1ec874717d8a

Added tag 4.6rc1 for changeset 1ec874717d8a

internals: correct capitalization of 'compression' stream level parameter

test-check-code: prevent from adding Python modules shadowed by ancient C

diffhelper: rename module to avoid conflicts with ancient C module (issue5846) Historically we had had C extensions in mercurial/, which shadows the pure Python modules of the same name forever unless we do clean build/install. I'm sloppy to think about new name, so just dropped the "s".

diffhelpers: backport 9e40bc4c1bde from C implementation 9e40bc4c1bde just says "harden testhunk." I don't think this would be the case, but it makes some sense to avoid negative index.