Set default in parameter expasion to avoid do-rebase exit due an unset var

The script has set -u so calling do-rebase --help fails with the following:

  $ ./do-rebase --help
  ./do-rebase: line 16: $1: unbound variable

Avoid this by setting a default value when doing the parameter expansion
for the positional parameter $1 passed to the usage() function.

Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>

do-rebase

@@ -13,7 +13,7 @@ alias formatpatch="othergit format-patch $format_patch_ops"
 usage()
 {
 	retcode=0
-	if [ -n "$1" ]; then
+	if [ -n "${1:-}" ]; then
 		exec 1>&2
 		retcode=$1
 	fi