Re: [PATCH v3 4/8] fetch: refactor the population of peer ref OIDs

Populate peer ref OIDs in get_ref_map instead of do_fetch. Besides
tightening scopes of variables in the code, this also prepares for
get_ref_map being able to be called multiple times within do_fetch.

get_ref_map() is only called in one place in builtin/fetch.c, and that
place is in do_fetch(), so moving functionality from do_fetch() to
get_ref_map() is perfectly fine, and also allows tightening of the scope
of the existing_refs variable.

Reviewed-by: Jonathan Tan <redacted>