cve-log: don't parse commit body for commit in mainline

I found a commit that use the "link" pattern to point out another
(boggus) commit. Also the "cherry-pick" pattern is probably used for
mainlined commit.

Therefore don't try to find mainline links in body of mainlined
commits.

Signed-off-by: Franck Bui-Huu <fbuihuu@gmail.com>
cve-log: catch some git formatting options that would break the rev parsing

Signed-off-by: Franck Bui-Huu <fbuihuu@gmail.com>
cve-import: die early if the passed file is not found

Signed-off-by: Franck Bui-Huu <fbuihuu@gmail.com>
cve-show: introduce "--format=format:" option

Signed-off-by: Franck Bui-Huu <fbuihuu@gmail.com>
cve-log: add support for 'format:' format specification

Signed-off-by: Franck Bui-Huu <fbuihuu@gmail.com>