bundle: refactor changegroup prune to be its own function
Moving the prune function to be a non-nested function allows extensions to
control which revisions are allowed in the changegroup. For example, in my
shallow repo extension I want to prevent filelogs from being added to the
bundle.
This also allows an extension to use a filelog implementation that doesn't
have revlog.linkrev implemented.
filectx: refactor filectx.rev() to use filectx._changeid
The code in filectx.rev() was identical to filectx._changeid. Fixing this
allows alternative filectx implementations to only override _changeid.