params/metrics: diff: implement tabulated markdown output (--show-md)

[skshetry]

Via --show-md on diffs.

• ❗ I have followed the Contributing to DVC checklist.

• 📖 If this PR requires documentation updates, I have created a separate PR (or issue, at least) in dvc.org and linked it here. If the CLI API is changed, I have updated tab completion scripts.

iterative/dvc.org#1266

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Thank you for the contribution - we'll try to review it as soon as possible. 🙏

• Still unsure about what should be shown in dvc diff --show-md.

The dvc params diff and similar commands would output header centered. With this change, it will be left-aligned.

Previously:

$ dvc params diff
   Path           Param        Old     New
params.yaml   lr               None   0.0041
params.yaml   process.bow      None   15000
params.yaml   process.thresh   None   0.98
params.yaml   train.epochs     None   70
params.yaml   train.layers     None   9

Now:

$ dvc params diff
Path         Param           Old    New
params.yaml  lr              None   0.0041
params.yaml  process.bow     None   15000
params.yaml  process.thresh  None   0.98
params.yaml  train.epochs    None   70
params.yaml  train.layers    None   9

$ dvc params diff --show-md
| Path        | Param          | Old   | New    |
|-------------|----------------|-------|--------|
| params.yaml | lr             | None  | 0.0041 |
| params.yaml | process.bow    | None  | 15000  |
| params.yaml | process.thresh | None  | 0.98   |
| params.yaml | train.epochs   | None  | 70     |
| params.yaml | train.layers   | None  | 9      |

^ Same output in markdown parsed by Github:

Path	Param	Old	New
params.yaml	lr	None	0.0041
params.yaml	process.bow	None	15000
params.yaml	process.thresh	None	0.98
params.yaml	train.epochs	None	70
params.yaml	train.layers	None	9

Resolve #3690

[skshetry]

This does not support aligning headers. It does have colalign but it aligns the whole column, and, I don't think it's a deal-breaker.

[dmpetrov]

@skshetry thank you for making the changes!
I added a bit more context - #3690 (comment)

[skshetry]

Should the output's column be swapped?

Path	Status
model.pkl	Added

[skshetry]

^ and also capitalized?

[efiop]

Should the output's column be swapped?

Currently, it is in the order in which it was requested. And, which matches git status.

^ and also capitalized?

not capitalized because it was requested in that form and matches git status.