script/backport-create-issue: handle long Redmine issue names

script/backport-create-issue: handle long Redmine issue names

Redmine issue names have a maximum length of 255 characters. When we prepend,
e.g., "nautilus: " to a very long issue name, we can end up with a string that
is over 255 characters long. Such a string will be refused by Redmine:

ValidationError: Subject is too long (maximum is 255 characters)

This patch avoids such an eventuality by truncating the post-prepend string to
255 characters.

Signed-off-by: Nathan Cutler <ncutler@suse.com>