branch: suggest <remote>/<branch> on upstream slip

When setting the upstream of the current branch to the 'main' branch
of the remote 'origin', i.e.,

    $ git branch --set-upstream-to origin/main

it is easy to mistakenly write

    $ git branch --set-upstream-to origin main

That is parsed as a request to set the upstream of the local branch
'main' to 'origin'. When 'main' does not exist, the command dies
with:

    fatal: branch 'main' does not exist

pointing at a branch the user never meant to name. When 'main' does
exist, it instead dies with:

    fatal: the requested upstream branch 'origin' does not exist

leaving the user equally puzzled.

When the operated-on branch is missing and '<remote>/<branch>' names
a real remote-tracking ref, suggest the intended form:

    $ git branch --set-upstream-to=origin/main

The suggestion is gated on '<remote>/<branch>' existing so it only
appears when a slipped slash is the likely explanation.

Signed-off-by: Harald Nordgren <haraldnordgren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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