diff: allow asking for diffs with no context

Previously, 0 meant default. This is problematic, as asking for 0
context lines is a valid thing to do.

Change GIT_DIFF_OPTIONS_INIT to default to three and stop treating 0
as a magic value. In case no options are provided, make sure the
options in the diff object default to 3.
Merge pull request #1175 from carlosmn/diff-0-ctx

Can't perform diff with no context lines