blame: fix object casting regression

Commit 1b0d400 refactored the prepare_final() function so
that it could be reused in multiple places. Originally, the
loop had two outputs: a commit to stuff into sb->final, and
the name of the commit from the rev->pending array.

After the refactor, that loop is put in its own function
with a single return value: the object_array_entry from the
rev->pending array. This contains both the name and the object,
but with one important difference: the object is the
_original_ object found by the revision parser, not the
dereferenced commit. If one feeds a tag to "git blame", we
end up casting the tag object to a "struct commit", which
causes a segfault.

Instead, let's return the commit (properly casted) directly
from the function, and take the "name" as an optional
out-parameter. This does the right thing, and actually
simplifies the callers, who no longer need to cast or
dereference the object_array_entry themselves.

[test case by Max Kirillov <max@max630.net>]

Signed-off-by: Jeff King <peff@peff.net>

t/annotate-tests.sh

@@ -68,6 +68,13 @@ test_expect_success 'blame 1 author' '
 	check_count A 2
 '
 
+test_expect_success 'blame by tag objects' '
+	git tag -m "test tag" testTag &&
+	git tag -m "test tag #2" testTag2 testTag &&
+	check_count -h testTag A 2 &&
+	check_count -h testTag2 A 2
+'
+
 test_expect_success 'setup B lines' '
 	echo "2A quick brown fox jumps over the" >>file &&
 	echo "lazy dog" >>file &&