completion: bash: improve function detection

1. We should quote the argument
 2. We don't need two redirections
 3. A safeguard for arguments (-a) would be good

Suggested-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -3359,7 +3359,7 @@ __git_support_parseopt_helper () {
 }
 
 __git_have_func () {
-	declare -f $1 >/dev/null 2>/dev/null
+	declare -f -- "$1" >/dev/null 2>&1
 }
 
 __git_complete_command () {