fetch --prune: Run prune before fetching

fetch --prune: Run prune before fetching

When we have a remote-tracking branch named "frotz/nitfol" from a
previous fetch, and the upstream now has a branch named "frotz",
fetch would fail to remove "frotz/nitfol" with a "git fetch --prune"
from the upstream. git would inform the user to use "git remote
prune" to fix the problem.

Change the way "fetch --prune" works by moving the pruning operation
before the fetching operation. This way, instead of warning the user
of a conflict, it autmatically fixes it.

Signed-off-by: Tom Miller <jackerran@gmail.com>
Tested-by: Thomas Rast <tr@thomasrast.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>