pull: clarify advice for the unconfigured error case

pull: clarify advice for the unconfigured error case

When pull --rebase fails because it cannot find what branch to
merge against, the error message implies we are trying to merge.
Say "rebase against" instead of "merge with" to avoid confusion.

The configuration suggested to remedy the situation uses a
confusing syntax, with variables specified in the dotted form
accepted by 'git config' but separated from their values by the
'=' delimiter used by config files.  Since the user will have to
edit this output anyway, it is more helpful to provide a config
file snippet to paste into an editor and modify.

Signed-off-by: Jan Krüger <jk@jk.gs>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>