Fix tests for membership to use 'not in'

flake8 reports:

  E713 test for membership should be 'not in'

Change-Id: I4446be67c431b7267105b53478d2ceba2af758d7
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/254451
Tested-by: David Pursehouse <dpursehouse@collab.net>
Reviewed-by: Mike Frysinger <vapier@google.com>

subcmds/branches.py

@@ -158,7 +158,7 @@ is shown, then the branch appears in all projects.
           for b in i.projects:
             have.add(b.project)
           for p in projects:
-            if not p in have:
+            if p not in have:
               paths.append(p.relpath)
 
         s = ' %s %s' % (in_type, ', '.join(paths))