ui-diff: Check the return value of get_sha1()

Sync with what we do everywhere else and check the return value of
get_sha1() instead of calling sha1_object_info() to validate the object.
Note that we later call lookup_commit_reference(), which checks that
both SHA1 values refer to commits, anyway.

Signed-off-by: Lukas Fleischer <cgit@cryptocrack.de>