Add `git-mergeable` script that verify that the current branch is mergeable in `master` (or in another one  branch)

# Use: git mergeable #to verify that the current checkouted branch is mergeable in 'master' (default branch)
# git mergeable myBranch #to verify that the current checkouted branch is mergeable in 'myBranch'

This script try to merge the branch and look if the merge is done without conflict
(and delete the temporary branch created for the check)