Yup, guilty as charged.
cf. <xmqqva9oe20y.fsf@gitster-ct.c.googlers.com>
"%s %s %s\n" with old and new commit object name and the message
will be "2 * len(hash_in_hex) + 4" bytes long (counting the three
whitespaces and the terminating NUL), and Shawn's original in
6de08ae6 ("Log ref updates to logs/refs/<ref>", 2006-05-17) actually
computed this one as "strlen(...) + 2*40+4". 100 was merely me being sloppier than Shawn at 8ac65937 ("Make sure
we do not write bogus reflog entries.", 2007-01-26), preferring
being sufficient over not wasting even a single byte.