Failure to colourise diffs if no `---` delimiter present

[Qeole]

Trying the add-on with this PR I noticed that we fail to colourise the patch for blocks where we don't have the --- delimiter. I think that if we don't have (j >= 0) after the for loop, we probably need to reset i to 0.

Originally posted by @Qeole in #120 (comment)