Manifezt: compare with merge-base

Context

The script manifezt.sh compares the current working directory with master to deduce which tests to run. @raphael-proust suggested to compare with git merge-base HEAD master instead, i.e. the common ancestor between master and the current branch. This gives better results when the current branch is not yet rebased on top of master.

Manually testing the MR

Cherry-pick the commit in a branch that is not rebased yet, and check that the following lists relevant tests:

make -C manifest manifest
manifest/manifezt.sh --list

Checklist

• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR