If no branch 'foo' exists but a tag 'foo' does, then
git merge foo^ results in
Merge branch 'foo' (early part)
as a commit message, because the relevant code path checks that
refs/heads/foo is a valid refname for writing rather than for
reading.
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>