Merge branch 'rs/usage-fallback-to-show-message-format'

vreportf(), which is usede by error() and friends, has been taught
to give the error message printf-format string when its vsnprintf()
call fails, instead of showing nothing useful to identify the
nature of the error.

* rs/usage-fallback-to-show-message-format:
  usage: report vsnprintf(3) failure

Author: gitster

usage.c

@@ -19,8 +19,11 @@ static void vreportf(const char *prefix, const char *err, va_list params)
 	}
 	memcpy(msg, prefix, prefix_len);
 	p = msg + prefix_len;
-	if (vsnprintf(p, pend - p, err, params) < 0)
+	if (vsnprintf(p, pend - p, err, params) < 0) {
+		fprintf(stderr, _("error: unable to format message: %s\n"),
+			err);
 		*p = '\0'; /* vsnprintf() failed, clip at prefix */
+	}
 
 	for (; p != pend - 1 && *p; p++) {
 		if (iscntrl(*p) && *p != '\t' && *p != '\n')