SCRIPTS: git-show-backports: make -m most only show the left branch

SCRIPTS: git-show-backports: make -m most only show the left branch

We've never used the output of the rightmost branch with this tool,
and it systematically causes two identical outputs making the job
harder during backport sessions. Let's simply remove the right part
when it's identical to the left one. This also adds a few line feeds
to make the output more readable.

(cherry picked from commit f456f6f2a36f0e2ce82ef80819974cb9dc8b4e86)
Signed-off-by: Willy Tarreau <w@1wt.eu>
(cherry picked from commit 2a53511500609a57c40bf0a33219954d45d6a31d)
Signed-off-by: Willy Tarreau <w@1wt.eu>