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>