annotate: Support annotation of files on other revisions.

This is a bug fix, and cleans up one or two other things spotted during the
course of tracking down the main bug here.

Also, the test-suite is updated to reflect this case.

Signed-off-by: Ryan Anderson <ryan@michonline.com>
(cherry picked from 2f7554b4db3ab2c2d3866b160245c91c9236fc9a commit)
Signed-off-by: Junio C Hamano <junkio@cox.net>

t/t8001-annotate.sh

@@ -6,4 +6,10 @@ test_description='git-annotate'
 PROG='git annotate'
 . ../annotate-tests.sh
 
+test_expect_success \
+    'Annotating an old revision works' \
+    '[ $(git annotate file master | awk "{print \$3}" | grep -c "^A$") == 2 ] && \
+     [ $(git annotate file master | awk "{print \$3}" | grep -c "^B$") == 2 ]'
+
+
 test_done