Add --no-ext-diff to diff
Add --no-ext-diff to diff

When users configure external diff tools, we show them each time we
invoke git diff internally (workflow for example).

We might add an option to use external diff on some circumstances,
for example git.destination. For now this is good enough.

Sadly I couldn't add a test for this. All my attempts failed. Git
ignored my fake $HOME or $XDG_CONFIG_HOME.

FIXED=132105839

Change-Id: Icc121e4b9555af65dc18a15e8f60585e64779422

google/copybara,google/copybara,google/copybara