completion: add --broken and --dirty to describe

When the flags for broken and dirty were implemented in
b0176ce (builtin/describe: introduce --broken flag, 2017-03-21)
and 9f67d2e (Teach "git describe" --dirty option, 2009-10-21)
the completion was not updated, although these flags are useful
completions. Add them.

Signed-off-by: Thomas Braun <thomas.braun@virtuell-zuhause.de>
Helped-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: t-b

contrib/completion/git-completion.bash

@@ -1385,7 +1385,7 @@ _git_describe ()
 		__gitcomp "
 			--all --tags --contains --abbrev= --candidates=
 			--exact-match --debug --long --match --always --first-parent
-			--exclude
+			--exclude --dirty --broken
 			"
 		return
 	esac