releasetools: Remove the global diff_done in blockimgdiff.py.

pylint complains about undefined `diff_done`:

W:754, 8: Global variable 'diff_done' undefined at the module level (global-variable-undefined)
W:820,14: Global variable 'diff_done' undefined at the module level (global-variable-undefined)

It would still warn about using global statement after adding the
definition.

W:859, 8: Using the global statement (global-statement)
W:925,14: Using the global statement (global-statement)

This CL computes 'diff_done' via 'len(diff_queue)' instead. It also
moves the progress reporting _before_ the diff work. This way it avoids
showing 100% progress with still changing filenames (because multiple
workers could see an empty queue simultaneously upon finishing their own
works).

There're possible alternatives, such as using the 'nonlocal' keyword in
Python 3 (which we're not there yet), or by using mutable object instead
(e.g. 'diff_done = [0]'). This CL looks cleaner, since it just kills the
var.

Test: Generate a BBOTA incremental. Check the on-screen progress
      report.
Test: `pylint --rcfile=pylintrc blockimgdiff.py` no longer complains
      about the global diff_done.
Change-Id: I339824735527e1f794b5b1dc99ff3fdb2da85744