bash: not all 'git bisect' subcommands make sense when not bisecting

... but only 'start' and 'replay'.  The other commands will either
error out or offer to start bisecting for the user.

Signed-off-by: SZEDER Gábor <szeder@ira.uka.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -907,7 +907,11 @@ _git_bisect ()
 	local subcommands="start bad good skip reset visualize replay log run"
 	local subcommand="$(__git_find_on_cmdline "$subcommands")"
 	if [ -z "$subcommand" ]; then
-		__gitcomp "$subcommands"
+		if [ -f "$(__gitdir)"/BISECT_START ]; then
+			__gitcomp "$subcommands"
+		else
+			__gitcomp "replay start"
+		fi
 		return
 	fi