rebase: allow "-" short-hand for the previous branch

Teach rebase the same shorthand as checkout and merge to name the
branch to rebase the current branch on; that is, that "-" means "the
branch we were previously on".

Requested-by: Tim Chase <git@tim.thechases.com>
Signed-off-by: Brian Gesiak <modocache@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/git-rebase.sh b/git-rebase.sh
index 8a3efa2..658c003 100755
--- a/git-rebase.sh
+++ b/git-rebase.sh
@@ -449,6 +449,10 @@
 		test "$fork_point" = auto && fork_point=t
 		;;
 	*)	upstream_name="$1"
+		if test "$upstream_name" = "-"
+		then
+			upstream_name="@{-1}"
+		fi
 		shift
 		;;
 	esac