merge: do not mistake (ancestor of) tag for branch

merge: do not mistake (ancestor of) tag for branch

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 <redacted>
Signed-off-by: Junio C Hamano <redacted>