fetch --prune: Always print header url

If "fetch --prune" is run with no new refs to fetch, but it has refs
to prune. Then, the header url is not printed as it would if there were
new refs to fetch.

Output before this patch:

	$ git fetch --prune remote-with-no-new-refs
	 x [deleted]         (none)     -> origin/world

Output after this patch:

	$ git fetch --prune remote-with-no-new-refs
	From https://github.com/git/git.git
	 x [deleted]         (none)     -> origin/test

Signed-off-by: Tom Miller <jackerran@gmail.com>
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". Prior
to this patch 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.

This patch changes the way "fetch --prune" works by moving the pruning
operation before the fetching operation. 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>