gettext: do not translate empty string

The gettext .po files have a header, but it looks like the
translation specification for an empty string.  This results in
_("") actually returning that header.

Check the input to _() and do not call gettext() on an empty string;
in some places, we run _(opts->help) where opts->help may be empty.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

gettext.h

@@ -44,6 +44,8 @@ extern int use_gettext_poison(void);
 
 static inline FORMAT_PRESERVING(1) const char *_(const char *msgid)
 {
+	if (!*msgid)
+		return "";
 	return use_gettext_poison() ? "# GETTEXT POISON #" : gettext(msgid);
 }