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
#require docutils gettext
$ $TESTDIR/check-gendoc pt_BR
checking for parse errors