narrow: use narrow_widen wireproto command to widen in case of ellipses Few releases ago, we introduce narrow_widen wireproto command to be used to widen narrow repositories. Before this patch, that was used in non-ellipses cases only. In ellipses cases, we still do exchange.pull() which can pull more data than required. After this patch, the client will first check whether server supports doing ellipses widening using wireproto command or not by checking server's wireproto capability. If the server is upto date and support latest ellipses capability, we call the wireproto command. Otherwise we fallback to exchange.pull() like before. The compat code make sure that things works even if one of the client or server is old. The initial version of this patch does not had this compat code. It's added to help Google release things smoothly internally. I plan to drop the compat code before the upcoming major release. Due to change to wireproto command, the code looks a bit dirty, next patches will clean that up. Differential Revision: https://phab.mercurial-scm.org/D6436