Fix --word-diff / --color-words handling

[dandavison]

Fixes #829 WIP

#807 was a pretty bad attempt at handling this correctly: while the colored content from git was emitted raw, in so doing they were not recognized as hunk lines and we lost line numbers and hunk headers.

[dandavison]

@mrjoel this branch fixes --word-diff / --color-words support, but, there's no support for line-numbers (or side-by-side) -- both will be silently ignored if --word-diff / --color-words input is detected.

For line-numbers the reason is that it would be a bit more work to implement: to calculate line numbers in the "before" and "after" state, delta normally relies on the - and + prefixes to indicate removed and added lines, but with --word-diff we don't have those and we'd have to detect wholly added/removed lines from the ANSI color sequences alone. In contrast, the implementation of --word-diff handling in this branch is based on processing all lines in the same way that unchanged (context) lines are usually processed.

For side-by-side the reason is that (a) one usually uses this with line numbers and (b) I don't think it makes much sense with --word-diff output since lines with both additions and removals will be the same in the left and right panels.

[dandavison]

@th1000s de37c99 adds a builder method DeltaTest::with_calling_process(command: &str) allowing your FakeParentArgs functionality to be accessed easily in DeltaTest. Hopefully you agree with the spirit of the commit if not the exact implementation :)