completion: teach ls-remote to complete options

ls-remote needs to complete remote names and its own options. In
addition to the existing remote name completions, do also complete
the options --heads, --tags, --refs, --get-url, and --symref.

Signed-off-by: Cornelius Weig <cornelius.weig@tngtech.com>
Reviewed-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -1449,6 +1449,12 @@ _git_ls_files ()
 
 _git_ls_remote ()
 {
+	case "$cur" in
+	--*)
+		__gitcomp "--heads --tags --refs --get-url --symref"
+		return
+		;;
+	esac
 	__gitcomp_nl "$(__git_remotes)"
 }