fetch, remote: properly convey --no-prune options to subprocesses

If --no-prune is passed to one of the following commands:

    git fetch --all
    git fetch --multiple
    git fetch --recurse-submodules
    git remote update

then it must also be passed to the "fetch" subprocesses that those
commands use to do their work.  Otherwise there might be a fetch.prune
or remote.<name>.prune configuration setting that causes pruning to
occur, contrary to the user's express wish.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/builtin/fetch.c b/builtin/fetch.c
index 1514b90..5ddb9af 100644
--- a/builtin/fetch.c
+++ b/builtin/fetch.c
@@ -936,8 +936,8 @@
 {
 	if (dry_run)
 		argv_array_push(argv, "--dry-run");
-	if (prune > 0)
-		argv_array_push(argv, "--prune");
+	if (prune != -1)
+		argv_array_push(argv, prune ? "--prune" : "--no-prune");
 	if (update_head_ok)
 		argv_array_push(argv, "--update-head-ok");
 	if (force)