"git pull --tags": error out with a better message.

"git pull --tags": error out with a better message.

When "git pull --tags" is run without any other arguments, the
standard error message "You told me to fetch and merge stuff but
there is nothing to merge!  You might want to fix your config"
is given.

While the error may be technically correct, fixing the config
would not help, as "git pull --tags" itself tells "git fetch"
not to use the configured refspecs.

This commit makes "git pull --tags" to issue a different error
message to avoid confusion.  This is merely an interim solution.

In the longer term, it would be a better approach to change the
semantics of --tags option to make "git fetch" and "git pull"
to:

 (1) behave as if no --tags was given (so an explicit refspec on
     the command line overrides configured ones, or no explicit
     refspecs on the command line takes configured ones); but

 (2) no auto-following of tags is made even when using
     configured refspecs; and

 (3) fetch all tags as not-for-merge entries".

Then we would not need to have this separate error message, as
the ordinary merge will happen even with the --tags option.

Signed-off-by: Junio C Hamano <gitster@pobox.com>