Add describe test.

... with help from Shawn.

Signed-off-by: Junio C Hamano <junkio@cox.net>

t/t6120-describe.sh

@@ -0,0 +1,97 @@
+#!/bin/sh
+
+test_description='test describe
+
+                       B
+        .--------------o----o----o----x
+       /                   /    /
+ o----o----o----o----o----.    /
+       \        A    c        /
+        .------------o---o---o
+                     D   e
+'
+. ./test-lib.sh
+
+check_describe () {
+	expect="$1"
+	shift
+	R=$(git describe "$@") &&
+	test_expect_success "describe $*" '
+	case "$R" in
+	$expect)	echo happy ;;
+	*)	echo "Oops - $R is not $expect";
+		false ;;
+	esac
+	'
+}
+
+test_expect_success setup '
+
+	test_tick &&
+	echo one >file && git-add file && git-commit -m initial &&
+	one=$(git-rev-parse HEAD) &&
+
+	test_tick &&
+	echo two >file && git-add file && git-commit -m second &&
+	two=$(git-rev-parse HEAD) &&
+
+	test_tick &&
+	echo three >file && git-add file && git-commit -m third &&
+
+	test_tick &&
+	echo A >file && git-add file && git-commit -m A &&
+	test_tick &&
+	git-tag -a -m A A &&
+
+	test_tick &&
+	echo c >file && git-add file && git-commit -m c &&
+	test_tick &&
+	git-tag c &&
+
+	git reset --hard $two &&
+	test_tick &&
+	echo B >side && git-add side && git-commit -m B &&
+	test_tick &&
+	git-tag -a -m B B &&
+
+	test_tick &&
+	git-merge -m Merged c &&
+	merged=$(git-rev-parse HEAD) &&
+
+	git reset --hard $two &&
+	test_tick &&
+	echo D >another && git-add another && git-commit -m D &&
+	test_tick &&
+	git-tag -a -m D D &&
+
+	test_tick &&
+	echo DD >another && git commit -a -m another &&
+
+	test_tick &&
+	git-tag e &&
+
+	test_tick &&
+	echo DDD >another && git commit -a -m "yet another" &&
+
+	test_tick &&
+	git-merge -m Merged $merged &&
+
+	test_tick &&
+	echo X >file && echo X >side && git-add file side &&
+	git-commit -m x
+
+'
+
+check_describe A-* HEAD
+check_describe A-* HEAD^
+check_describe D-* HEAD^^
+check_describe A-* HEAD^^2
+check_describe B HEAD^^2^
+
+check_describe A-* --tags HEAD
+check_describe A-* --tags HEAD^
+check_describe D-* --tags HEAD^^
+check_describe A-* --tags HEAD^^2
+check_describe B --tags HEAD^^2^
+
+test_done