releasetools: Fix the detection of using squashfs.

We should disable using imgdiff if *any* of the source and target
partitions uses squashfs.

Bug: 30004734
Test: Create an incremental with two builds with one of them uses squashfs.
Change-Id: I826cd13d7b852c548e4b45e61f5ae00f6407cac3
1 file changed