Suggestions for improvement when dealing with remote branch shuffling

* it looks like "git remote prune" on any remote may be a good candidate to be
included in "git gc" (although probably only for the candidate refs seeing their
last reflog entry removed)

* when a remote had a "foo/bar" ref in the past, still available locally as
"remotes/origin/foo/bar" because the user did not think about running "remote
prune origin", and a new remote branch named "foo" appears, fetch refuses to
update the local "remotes/origin/foo" ref, but does not give the reason to the
user. OTOH, another user reported fetch suggesting running "remote prune origin"
- but he may have had a slightly different history of fetch runs (seen in
1.6.5.1 msys). Probably this message should also be issued in the 1st case.