tag: keep the message file in case ref transaction fails

tag: keep the message file in case ref transaction fails

The ref transaction can fail after the user has written their tag
message. In particular, if there exists a tag `foo/bar` and `git tag -a
foo` is said then the command will only fail once it tries to write
`refs/tags/foo`, which is after the file has been unlinked.

Hold on to the message file for a little longer so that it is not
unlinked before the fatal error occurs.

Signed-off-by: Kristoffer Haugsbakk <code@khaugsbakk.name>
Signed-off-by: Junio C Hamano <gitster@pobox.com>