Be a bit less strict about separate branches for each change

Basically, there might very well be valid reasons to roll one change
into another if one is dependant on the other, even if they're
technically distinctly different changes. Ideally this shouldn't
happen, but let's at least allow it if really needed.

common-flow.md

@@ -65,8 +65,8 @@ interpreted as described in [RFC 2119](https://tools.ietf.org/html/rfc2119).
        history.
     4. You SHOULD regularly push your work to the same named branch on the
        remote server.
-    5. You MUST create separate change branches for each distinctly different
-       change. You MUST NOT include multiple unrelated changes into a single
+    5. You SHOULD create separate change branches for each distinctly different
+       change. You SHOULD NOT include multiple unrelated changes into a single
        change branch.
     6. When a change branch is created, the branch that it is created from
        SHOULD be referred to as the "source branch". Each change branch also