Do not add --all-features to combinations if it is already covered by another combination

[taiki-e]

This fixes an issue where --feature-powerset and --each-feature add --all-features as one of the combinations, even if it is already covered by another combination. (The initial work on this fix was included in #140.)

Hopefully, the new behavior should be explained almost exactly in the long help message.

cargo-hack/tests/long-help.txt

Lines 30 to 37 in 4234b37

	--each-feature
	Perform for each feature of the package.
	This also includes runs with just --no-default-features flag, and default features.
	When this flag is not used together with --exclude-features (--skip) and
	--include-features and there are multiple features, this also includes runs with just
	--all-features flag.

cargo-hack/tests/long-help.txt

Lines 39 to 47 in 4234b37

	--feature-powerset
	Perform for the feature powerset of the package.
	This also includes runs with just --no-default-features flag, and default features.
	When this flag is used together with --depth or namespaced features (-Z
	namespaced-features) and not used together with --exclude-features (--skip) and
	--include-features and there are multiple features, this also includes runs with just
	--all-features flag.

Follow-up: #42

[taiki-e]

bors r+

[bors]

Build succeeded:

• ci