This will only have an effect if one's buildtools checkout is new enough, since
only then:

a) clang-format will be new enough to do a decent job for java files
b) clang-format-diff.py will be new enough to look at java files

So for older buildtools checkouts, `git cl format` will continue to ignore java
files.  For newer buildtools checkouts (like the one pulled by `gclient sync`
with a recent chromium checkout), it'll work and do a fairly decent job.

BUG=429372

Review URL: https://codereview.chromium.org/788933003

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@293312 0039d316-1c4b-4281-b951-d872f2087c98