sequencer: ensure labels that are object IDs are rewritten

sequencer: ensure labels that are object IDs are rewritten

When writing the todo script for --rebase-merges, we try to find a label
for certain commits.  If the label ends up being a valid object ID, such
as when we merge a detached commit, we want to rewrite it so it is no
longer a valid object ID.

However, the code path that does this checks for its length to be
equivalent to GIT_SHA1_RAWSZ, which isn't correct, since what we are
reading is a hex object ID.  Instead, check for the length being
equivalent to that of a hex object ID.  Use the_hash_algo so this code
works regardless of the hash size.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>