config: Give error message when not changing a multivar

config: Give error message when not changing a multivar

When trying to set a multivar with "git config var value", "git config"
issues

warning: remote.repoor.push has multiple values

leaving the user under the impression that the operation succeeded,
unless one checks the return value.

Instead, make it

warning: remote.repoor.push has multiple values
error: cannot overwrite multiple values with a single value
       Use a regexp, --add or --set-all to change remote.repoor.push.

to be clear and helpful.

Note: The "warning" is raised through other code paths also so that it
needs to remain a warning for these (which do not raise the error). Only
the caller can determine how to go on from that.

Signed-off-by: Michael J Gruber <git@drmicha.warpmail.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>