Comment 14 for bug 1565711

It also passes on Vivid and Wily, so looking at changes between ifupdown-0.7.47.2ubuntu4.4 (trusty-updates) and ifupdown-0.7.48.1ubuntu10 (vivid). These looks particularly relevant:

+ifupdown (0.7.48.1ubuntu1) utopic; urgency=medium
+
+ [ Stéphane Graber ]
+ * Merge from Debian. Remaining changes:

+ - Allow setting the MTU and HWADDR on manual interfaces. (LP: #1294807)
+ - The above change also means that manual interfaces will now be
+ brought up and down (ias many users expected until now).
+ - Disable link.defn as it's not covering all the cases supported by
+ the vlan and bridge hooks and so causes more harm than good at this
+ point. (LP: #1295304)