narrow: factor out logic to create cg while widening into separate fn This patch takes out the logic which generates a changegroup for widening a narrow clone when ellipses are disabled. This is done because future patches will introduce a narrow_widen() wireprotocol command which will send a bundle2 with changegroup and will use this function. The new function for now returns just the changegroup for compatibility with existing code, but in future patches, when we establish a wireprotocol command and call this function from there, this will return the required bundle2. Differential Revision: https://phab.mercurial-scm.org/D4786