[git] fetch from origin with deleted remote branches?

Regarding git fetch -p, its behavior changed in Git 1.9, and only Git 2.9.x/2.10 reflects that.

See commit 9e70233 (13 Jun 2016) by Jeff King (peff).
(Merged by Junio C Hamano -- gitster -- in commit 1c22105, 06 Jul 2016)

fetch: document that pruning happens before fetching

This was changed in 10a6cc8 (fetch --prune: Run prune before fetching, 2014-01-02), but it seems that nobody in that discussion realized we were advertising the "after" explicitly.

So the documentation now states:

Before fetching, remove any remote-tracking references that no longer exist on the remote

That is because:

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 automatically fixes it.