Enable bash completions on Macs

If they installed bash-completions from homebrew, this should make our
completions work.

bash_completions.go

@@ -19,7 +19,6 @@ const (
 func preamble(out *bytes.Buffer) {
 	fmt.Fprintf(out, `#!/bin/bash
 
-
 __debug()
 {
     if [[ -n ${BASH_COMP_DEBUG_FILE} ]]; then
@@ -27,6 +26,14 @@ __debug()
     fi
 }
 
+# Homebrew on Macs have version 1.3 of bash-completion which doesn't include
+# _init_completion. This is a very minimal version of that function.
+__my_init_completion()
+{
+    COMPREPLY=()
+    _get_comp_words_by_ref cur prev words cword
+}
+
 __index_of_word()
 {
     local w word=$1
@@ -188,7 +195,11 @@ func postscript(out *bytes.Buffer, name string) {
 	fmt.Fprintf(out, "__start_%s()\n", name)
 	fmt.Fprintf(out, `{
     local cur prev words cword
+    if declare -F _init_completions >/dev/null 2>&1; then
         _init_completion -s || return
+    else
+        __my_init_completion || return
+    fi
 
     local c=0
     local flags=()
@@ -292,7 +303,7 @@ func writeRequiredFlag(cmd *Command, out *bytes.Buffer) {
 	fmt.Fprintf(out, "    must_have_one_flag=()\n")
 	flags := cmd.NonInheritedFlags()
 	flags.VisitAll(func(flag *pflag.Flag) {
-		for key, _ := range flag.Annotations {
+		for key := range flag.Annotations {
 			switch key {
 			case BashCompOneRequiredFlag:
 				format := "    must_have_one_flag+=(\"--%s"