SCRIPTS: make git-show-backports capable of limiting its history

SCRIPTS: make git-show-backports capable of limiting its history

When comparing very different branches, it can take a very long time
to scan all commits from the very old common ancestor (eg: haproxy
1.4 to 1.7). Now it is possible to specify a range of commits instead
of a specific branch, and the analysis will be limited to this range
for all commits. The user is responsible for ensuring that the range
covers all possible backports from base to ref, otherwise some of them
may be reported missing while they are not.

This also works with linux kernels, for example :

   git-show-backports -u -q -m -r v3.14.69 -b v3.14.65 v3.10.101..HEAD