Clarify help message when no remote is specified in fetch/pull.

Clarify help message when no remote is specified in fetch/pull.

The message is especially confusing when "git fetch" is ran from "git
pull", for users not aware of "git fetch". The new message makes it clear
that "fetch" means "fetch new revisions", and gives hint on the solution.

We don't add a advice.* configuration option since this message doesn't
appear in normal use, and shouldn't disturb advanced users.

Signed-off-by: Matthieu Moy <redacted>
Signed-off-by: Junio C Hamano <redacted>