git-fetch: fail if specified refspec does not match remote.

git-fetch: fail if specified refspec does not match remote.

'git-fetch remote no-such-ref' succeeded without fetching any
ref from the remote.  Detect such case and report an error.

Note that this makes 'git-fetch remote master master' to fail,
because the remote branch 'master' matches the first refspec,
and the second refspec is left unmatched, which is detected by
the error checking logic.  This is somewhat unintuitive, but
giving the same refspec more than once to git-fetch is useless
in any case so it should not be much of a problem.  I'd accept a
patch to change this if somebody cares enough, though.

Signed-off-by: Junio C Hamano <junkio@cox.net>