bash-completion: add git-prune into bash completion

Sometimes git would suggest the user to run `git prune` when there are
too many unreachable loose objects. It's more user-friendly if we add
git-prune into bash completion.

Signed-off-by: John Lin <johnlinp@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

command-list.txt

@@ -136,7 +136,7 @@ git-pack-redundant                      plumbinginterrogators
 git-pack-refs                           ancillarymanipulators
 git-parse-remote                        synchelpers
 git-patch-id                            purehelpers
-git-prune                               ancillarymanipulators
+git-prune                               ancillarymanipulators   complete
 git-prune-packed                        plumbingmanipulators
 git-pull                                mainporcelain           remote
 git-push                                mainporcelain           remote