pretty: add %(describe)

Add a format placeholder for describe output.  Implement it by actually
calling git describe, which is simple and guarantees correctness.  It's
intended to be used with $Format:...$ in files with the attribute
export-subst and git archive.  It can also be used with git log etc.,
even though that's going to be slow due to the fork for each commit.

Suggested-by: Eli Schwartz <eschwartz@archlinux.org>
Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t4205-log-pretty-formats.sh

@@ -962,4 +962,14 @@ test_expect_success 'log --pretty=reference is colored appropriately' '
 	test_cmp expect actual
 '
 
+test_expect_success '%(describe) vs git describe' '
+	git log --format="%H" | while read hash
+	do
+		echo "$hash $(git describe $hash)"
+	done >expect &&
+	git log --format="%H %(describe)" >actual 2>err &&
+	test_cmp expect actual &&
+	test_must_be_empty err
+'
+
 test_done