diff: use `context.diff` to produce diff
We want to make sure `context.diff` if full featured. Using it in the main
diff code path is a good way to do so.
The end goal is to be able to easily compute diff between in-memory context.
# See http://EditorConfig.org for the specification
root = true
[*.py]
indent_size = 4
indent_style = space
trim_trailing_whitespace = true
[*.{c,h}]
indent_size = 8
indent_style = tab
trim_trailing_whitespace = true
[*.t]
indent_size = 2
indent_style = space
trim_trailing_whitespace = false