Make the status string shown for subthreads consistent

As pointed out by @linquize, we are not being consistent here: https://github.com/google/git-appraise/commit/f3553772ced2ab9a0aec289404f1a1cb62950f85#diff-9c7a05e4295bdceebccd9558859ab151R123

In one case we use the acronym "lgtm" and in the other we spell it out as "needs work". We should pick one approach (acronyms or spelled-out) and be consistent with it.