Inline repo_remote_fetch_summary

Inline repo_remote_fetch_summary

This was only used in one place, and (especially with the simplification
with GMainContextPopDefault) and the one caller doesn't really do
much more than call the helper. Additionally, what little it does (saving
the result in the cache) is inherently tied to how the helper work,
and will become even more so when we support summary indexes.

This is a preparatory cleanup for supporting summary indexes. It
doesn't change any behaviour and passes make check on its own.