Now that update_engine no longer uses minor version 1, old file and
new file are always different in the device, so we can write the
output to disk right away to save memory.
The old behavior is still kept if the new file is overlapping with the
old file for paycheck.py usage.

Test: bsdiff_unittest
Test: apply a 3M patch overwriting old file, uses 25M memory.
Test: apply a 3M patch not overwriting old file, uses 9M memory.
Bug: 25773600

Change-Id: Ic289c9bcc6f1810d0f222a9a4241c05756084b1c