exp run: do nothing if result matches parent (HEAD)

[pmrowla]

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Related to #5593

If the result of exp run is identical to the parent commit (HEAD), do nothing. Previously, for --temp runs we would do nothing, but workspace runs would generate an empty named experiment which was identical to HEAD.

[skshetry]

@pmrowla, does this also solve #5567?

[pmrowla]

@skshetry possibly? There was a bug with workspace runs where the workspace was considered dirty because of the way we pass repro args into the experiment (even though the end result would still be an empty git commit) that should be resolved by this PR.

That may be what was causing the "conflicts with existing experiment" issue you were seeing in that issue

[skshetry]

@skshetry possibly?

Looks like not.

cd "$(mktemp -d)" && git init && dvc init
dvc stage add -n echo -p foo -m metrics.yaml "echo params.yaml > metrics.yaml"
echo "foo: 10" > params.yaml
git add -A && git commit -m "initialize"
dvc exp run
dvc exp run