Add clap cosmetic argument names

[dandavison]

No description provided.

[th1000s]

I for my part find these SCREAMING_SNAKE_CASE default specifiers visually jarring and yes, I know Pythons argparse and now clap use them by defaults. Its ok when it is just a short N or NUM, but these longer ones... but I see you try to shorten them to reduce the --for-bar <FOO_BAR> cases.

[dandavison]

I for my part find these SCREAMING_SNAKE_CASE default specifiers visually jarring

Agreed. What do you think the best solution is?

1. Disable them entirely
   I'm not sure if it's possible. E.g. value_name can't be set to None. And if we could, I worry that it would be a problem that user's wouldn't know which options required associated values.
2. Lower case
   I worry that they're a bit hard to visually separate from the (important) option name
3. Use a different color
   It may be possible but I haven't found how yet.

Also using upper case allows us to make this sort of informal distinction where here we are giving the allowed variants literally:

--paging <auto|always|never>

I think it's it at least a bit better now in this PR -- before they were screaming back the option name verbatim.

[dandavison]

Incidentally, check out the concise help via delta -h in this branch -- I think it's going to be useful.

[dandavison]

Merging this one which uses a smaller set of shorter names (e.g. STYLE_STRING many times over) but I'd be happy if we can identify further improvements.

[th1000s]

informal distinction

That's quite formal already and a good reason. I can also see that these are easier to visually separate.

I'd probably replace

STYLE_STRING => STYLE
FORMAT_STRING and other *FORMAT* => FMT
NUMBER_OF_SPACES / WIDTH => N

check out the concise help via delta -h

I couldn't help but notice this breaking the terminal width :)

    --merge-conflict-theirs-diff-header-decoration-style <MERGE_CONFLICT_THEIRS_DIFF_HEADER_DECORATION_STYLE>    Style string for the decoration of the header above the 'theirs' merge conflict diff [default: box]

[dandavison]

I'd probably replace

OK, I think I like those. People just have to understand once, but they are forever after condemned to look at them...

I couldn't help but notice this breaking the terminal width :)

Right, I found I couldn't change that one for some reason. I reported a bug against clap: clap-rs/clap#3300 but it's of course entirely possible I'm missing something.