metrics/params: diff: don't show diff if active branch is the same as workspace

[dberenbaum]

Before, if you did dvc metrics diff main from the main branch, you would get:

$ dvc metrics diff main
Path                   Metric      main     workspace    Change
training/metrics.json  step        14       -            -
training/metrics.json  test.acc    0.7735   -            -
training/metrics.json  test.loss   0.95962  -            -
training/metrics.json  train.acc   0.7694   -            -
training/metrics.json  train.loss  0.9731   -            -

This made it look like the metrics were missing in the workspace and there was some difference from main even in a clean repo state.

After this PR, there will be no output from the command above.

[codecov]

Codecov Report

Base: 93.55% // Head: 93.54% // Decreases project coverage by -0.00% ⚠️

Coverage data is based on head (ed689da) compared to base (1d5de9c).
Patch coverage: 93.54% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8716      +/-   ##
==========================================
- Coverage   93.55%   93.54%   -0.01%     
==========================================
  Files         457      457              
  Lines       36237    36254      +17     
  Branches     5260     5262       +2     
==========================================
+ Hits        33900    33914      +14     
- Misses       1832     1835       +3     
  Partials      505      505              

Impacted Files	Coverage Δ
dvc/repo/metrics/show.py	95.40% <71.42%> (+0.05%)	⬆️
dvc/repo/metrics/diff.py	100.00% <100.00%> (ø)
dvc/repo/params/diff.py	100.00% <100.00%> (ø)
dvc/repo/params/show.py	94.00% <100.00%> (+0.06%)	⬆️
tests/func/metrics/test_diff.py	100.00% <100.00%> (ø)
tests/func/params/test_diff.py	100.00% <100.00%> (ø)
tests/unit/repo/experiments/conftest.py	90.62% <0.00%> (-4.69%)	⬇️

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

☔ View full report at Codecov.
📢 Do you have feedback about the report comment? Let us know in this issue.

[efiop]

Does seem pretty weird that show doesn't return it at all. Looks like https://github.com/iterative/dvc/blob/0e5674ffeabb4f911a9486da8ff3ba3a33d1d98c/dvc/repo/metrics/show.py#L145 is the metrics show-specific behaviour that leaked into metrics diff. Probably needs a flag not to hide workspace metrics or it should hide it in CLI (that would be my expectation). Taking another look at #3025 ...

[efiop]

@dberenbaum @daavoo Guys, do you find this https://github.com/iterative/dvc/blob/0e5674ffeabb4f911a9486da8ff3ba3a33d1d98c/dvc/repo/metrics/show.py#L145 behaviour useful at all in the API? I'm kinda tempted to move that hiding logic to the CLI layer so we don't have to fix it in diff at all. If having that in API is useful, then I'll make it an opt-out flag, so again we don't have to fix it in diff. I guess the latter is the simplest one anyway...

[efiop]

Added a flag for now.