Teach the "@{-1} syntax to "git branch"

This teaches the new "@{-1} syntax to refer to the previous branch to "git
branch".  After looking at somebody's faulty patch series on a topic
branch too long, if you decide it is not worth merging, you can just say:

    $ git checkout master
    $ git branch -D @{-1}

to get rid of it without having to type the name of the topic you now hate
so much for wasting a lot of your time.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t0100-previous.sh

@@ -0,0 +1,26 @@
+#!/bin/sh
+
+test_description='previous branch syntax @{-n}'
+
+. ./test-lib.sh
+
+test_expect_success 'branch -d @{-1}' '
+	test_commit A &&
+	git checkout -b junk &&
+	git checkout - &&
+	test "$(git symbolic-ref HEAD)" = refs/heads/master &&
+	git branch -d @{-1} &&
+	test_must_fail git rev-parse --verify refs/heads/junk
+'
+
+test_expect_success 'branch -d @{-12} when there is not enough switches yet' '
+	git reflog expire --expire=now &&
+	git checkout -b junk2 &&
+	git checkout - &&
+	test "$(git symbolic-ref HEAD)" = refs/heads/master &&
+	test_must_fail git branch -d @{-12} &&
+	git rev-parse --verify refs/heads/master
+'
+
+test_done
+