push: avoid showing false negotiation errors

When "git push" is configured to use the push negotiation, a push of
deletion of a branch (without pushing anything else) may end up not
having anything to negotiate for the common ancestor discovery.

In such a case, we end up making an internal invocation of "git
fetch --negotiate-only" without any "--negotiate-tip" parameters
that stops the negotiate-only fetch from being run, which by itself
is not a bad thing (one fewer round-trip), but the end-user sees a
"fatal: --negotiate-only needs one or more --negotiation-tip=*"
message that the user cannot act upon.

Teach "git push" to notice the situation and omit performing the
negotiate-only fetch to begin with.  One fewer process spawned, one
fewer "alarming" message given the user.

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

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