checkpatch: avoid error on cover letter files

checkpatch: avoid error on cover letter files

Running checkpatch on a directory that contains a cover letter reports
this error:

    Checking /tmp/tmpbnngauy3/0000-cover-letter.patch...
    ERROR: Does not appear to be a unified-diff format patch

    total: 1 errors, 0 warnings, 0 lines checked

Let's skip cover letter as it is already done in the Linux kernel
commits 06330fc40e3f ("checkpatch: avoid NOT_UNIFIED_DIFF errors
on cover-letter.patch files") and a08ffbef4ab7 ("checkpatch: fix
ignoring cover-letter logic").

Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Message-Id: <20200917170212.92672-1-sgarzare@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>