subprocess: add a function to shell quote a single string

We currently have a function to shell quote a list of strings, but in
some cases we may want to just quote a single string.  To avoid the
complexity of having to create a bunch of temporaries and then index out
the result, create a function that shell quotes just a single string.

subprocess/subprocess.go

@@ -86,18 +86,22 @@ func Output(cmd *Cmd) (string, error) {

var shellWordRe = regexp.MustCompile(`\A[A-Za-z0-9_@/.-]+\z`)

// ShellQuoteSingle returns a string which is quoted suitably for sh.
func ShellQuoteSingle(str string) string {
	// Quote anything that looks slightly complicated.
	if shellWordRe.FindStringIndex(str) == nil {
		return "'" + strings.Replace(str, "'", "'\\''", -1) + "'"
	}
	return str
}

// ShellQuote returns a copied string slice where each element is quoted
// suitably for sh.
func ShellQuote(strs []string) []string {
	dup := make([]string, 0, len(strs))

	for _, str := range strs {
		// Quote anything that looks slightly complicated.
		if shellWordRe.FindStringIndex(str) == nil {
			dup = append(dup, "'"+strings.Replace(str, "'", "'\\''", -1)+"'")
		} else {
			dup = append(dup, str)
		}
		dup = append(dup, ShellQuoteSingle(str))
	}
	return dup
}