github-merge-pr: introduce new script to merge pr on github

github-merge-pr: introduce new script to merge pr on github

Introduce a new script github-merge-pr.sh to facilitate merging pr from
github by flagging them as actually merged.

Assuming you are in the github repo.

Usage: github-merge-pr.sh <PR-ID> [rebase-branch] [dry-run]

PR-ID is PR #ID
rebase-branch is master to be rebased on (master,openwrt2..03...)
dry-run doesn't execute anything and is used to check what command will
be executed

Signed-off-by: Christian Marangi <[email protected]>