Use triple-dot do diff YAML in CI

The current diff mechanism in rosdep_repo_check uses a 'bare' diff with
the configured upstream ref. If the upstream ref contains additional
commits which are not present in the PR's HEAD which remove lines of
YAML, this bare diff will report those lines as 'added' when comparing
the PR to the target, which is certainly not what we want to do.

We can use the git triple-dot notation to compare only commits which are
present in HEAD but not in target, effectively comparing between the
PR's HEAD and the merge base with the target branch, instead of the
target branch directly.

One disadvantage of this approach is that uncommitted changes are no
longer considered part of the diff. For this reason, I'm only adding the
triple dot during diffs with the specifically-configured upstream branch
and not the 'fallback' comparison with origin/master. This way, you can
still invoke the test locally without the need to commit the changes
first.

test/rosdep_repo_check/test_rosdep_repo_check.py

@@ -70,7 +70,7 @@ def get_changed_line_numbers():
         # Check url
         assert remote_url == DIFF_REPO, \
             '%s remote url [%s] is different than %s' % (UPSTREAM_NAME, remote_url, DIFF_REPO)
-        base_ref = '%s/%s' % (UPSTREAM_NAME, DIFF_BRANCH)
+        base_ref = '%s/%s...' % (UPSTREAM_NAME, DIFF_BRANCH)
     except subprocess.CalledProcessError:
         # No remote so fall back to origin/master
         print('WARNING: No remote %s detected, falling back to origin master. Make sure it is up to date.' % UPSTREAM_NAME, file=sys.stderr)