Bug 948787 - Print diffs during config.status; r=glandium

Build system developers commonly need to see what changes have on the
generated build files. We often put our objdir under version control and
diff commits before and after running config.status.

This patch adds a --diff option to config.status that will print diffs
of changes made during config.status. This functionality is implemented
on top of FileAvoidWrite, using Python's built-in diffing library.

While display of diffs is opt-in, diffs are always being captured when
config.status runs. There could be an unwanted performance regression
from this. Because diffs are only computed if files change and most
files don't change during most config.status runs, this greatly reduces
the surface area of the concern. The area for largest concern is clobber
builds. On my machine, I measured an increase of 0.2 to 0.3s from 2.0s.
While this is 10-15%, the total time is so small that I don't feel
snaking a "capture diff" flag through the build system is worth the
effort. This would make a decent followup bug if this turns out to be a
problem in the future.

I also snuck in a change to reindent all-tests.json because displaying
diffs for this massive 11MB all-in-one-line JSON file results in an
extremely large string being printed to my terminal.

--HG--
extra : rebase_source : c0f7ff69cad282e63a050e67f156dbe96b49a142