Protect against format.coverLetter=on in git_format_patch()

If the user's ~/.gitconfig has format.coverLetter=on then
git-format-patch(1) always creates 0000-cover-letter.patch.  Explicitly
pass --no-cover-letter to prevent that from happening.

Signed-off-by: Stefan Hajnoczi <stefanha@gmail.com>
Fall back to format.coverLetter

Use format.coverLetter as the default if there is no higher priority
factor influencing the git-format-patch(1) --cover-letter option.

Also increase the priority of gitpublishprofile.<profile>.message since
it is more specific than format.coverLetter.

Signed-off-by: Stefan Hajnoczi <stefanha@gmail.com>