contruib: ability to set a prefix on backport script

When pushing a PR it might be handy to set the [skip ci] flag if we know
upfront the content should not trigger the CI.

Now you can add [skip ci] as $4 in your command line.

Signed-off-by: Sébastien Han <seb@redhat.com>

contrib/backport_to_stable_branch.sh

@@ -89,7 +89,7 @@ checkout
 cherry_pick
 push
 create_pr <<MSG
-Backport of ${3} in $stable_branch
+${4} Backport of ${3} in $stable_branch
 
 Backport of #${3} in $stable_branch
 MSG