changegroup: introduce "raw" versions of some commands
The commands getchangegroup, getlocalchangegroup and getsubset now each
have a version ending in -raw. The raw versions return the chunk generator
from the changegroup packer directly, without wrapping it in a chunkbuffer
and unpacker. This avoids extra chunkbuffers in the bundle2 code path.
Also, the raw versions can be extended to support alternative packers
in the future, to be used from bundle2.
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