devtools: fix version search with git < 2.7.0

devtools: fix version search with git < 2.7.0

The script git-log-fixes.sh (used in check-git-log.sh) looks
for git tags to find the version where a bug is introduced.

In DPDK 17.08, the script has been fixed to ignore tags from
non current branch.
It was using the option --merged which was introduced in git 2.7.0.

As git 2.7.0 is not so old, a fallback is provided for some years.

The fallback is replacing the tag --merged option by a branch filter.
If the tag is found in the branch, the branch name is replaced
by the tag.

This script could be improved to allow using another reference branch,
instead of hard coding HEAD branch (the current one).

Fixes: 26857dabb3c9 ("devtools: ignore non merged tags for backport")
Cc: stable@dpdk.org
Signed-off-by: Thomas Monjalon <thomas@monjalon.net>
Tested-by: Nelio Laranjeiro <nelio.laranjeiro@6wind.com>