devtools: ignore non merged tags for backport
devtools: ignore non merged tags for backport

When checking if a buggy commit was introduced in an old version,
the script compares last tag containing the bug and current version.
The non merged tags from non related branches must be ignored.

Signed-off-by: Thomas Monjalon <5f50a84c1fa3bcff146405017f36aec1a10a9e38@monjalon.net>

john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk