Stop the stability checker running on multiple commits on master.

The stability checker tried to work out which commits to test by
looking at all the commits on the current branch that aren't on
another branch. But if the current branch is e.g. master then this
will produce many more commits than intended. To fix this use the
merge-base with master when it's closer to HEAD than the last commit
not on another branch.

check_stability.py

@@ -408,6 +408,18 @@ def get_branch_point(user):
         commits = git("rev-list", "HEAD", *not_heads).split("\n")
         first_commit = commits[-1]
         branch_point = git("rev-parse", first_commit + "^")
+        # The above can produce a too-early commit if we are e.g. on master and there are
+        # preceding changes that were rebased and so aren't on any other branch. To avoid
+        # this issue we check for the later of the above branch point and the merge-base
+        # with master
+        merge_base = git("merge-base", "HEAD", "origin/master")
+        if (branch_point != merge_base and
+            not git("log", "--oneline", "%s..%s" % (merge_base, branch_point)).strip()):
+            logger.debug("Using merge-base as the branch point")
+            branch_point = merge_base
+        else:
+            logger.debug("Using first commit on another branch as the branch point")
+
     logger.debug("Branch point from master: %s" % branch_point)
     return branch_point