fix: skip --reuse-values for a release that is not installed (#481)

[rishabhvenu]

helm diff upgrade --install --reuse-values fails with Error: Failed to render chart: exit status 1 when the release does not exist yet. The same happens with --allow-unreleased and with --reset-then-reuse-values. helm upgrade --install --reuse-values works in that situation.

Cause: template() in cmd/helm.go runs helm get values whenever one of the two flags is set. Only the implicit reuse of values (no --set/--values given) has been limited to upgrades since a25763a. For a new install there is no release, so helm get values exits with release: not found and the render is aborted.

Fix: only fetch the existing values when the release exists. For a new install the two flags are ignored, which is what helm upgrade --install does. Nothing changes for a release that exists.

Tests:

• TestUpgradeCommand_Execution_ReuseValues runs the upgrade command against the fake helm. A new unreleased fake helm mode makes every helm get fail with release: not found. Six cases for a missing release fail on master with the error from the issue and pass with this change. They also check that helm get values is not called. Four cases for an existing release check that it still is.
• I ran the built binary with Helm v3.17.3 and v4.3.0 against a local kube-apiserver and etcd (no kubelet) with a ConfigMap-only chart. On master, --install or --allow-unreleased with --reuse-values or --reset-then-reuse-values fails, also together with --dry-run=server, --three-way-merge and HELM_DIFF_USE_UPGRADE_DRY_RUN=true. With this change each of them prints the manifest as added. For an installed release, --reuse-values --set other=changed gives the same diff before and after, with the previously set value kept.
• make format, make lint, make test and make verify-readme pass, and golangci-lint v2.14.0 reports 0 issues.

Not tested: a kind cluster, so the CI integration job has not been run locally. I did not add a scripts/issues/481.sh; I can add one if you want it.

Two things this does not change or changes on purpose:

• With Helm older than v3.14, --reset-then-reuse-values on a release that does not exist no longer returns the "requires at least helm version 3.14.0" error, because the flag is now ignored for a new install.
• The issue also suggests passing --reuse-values through to helm upgrade when HELM_DIFF_USE_UPGRADE_DRY_RUN is set. That is not part of this PR. In that mode the values of an existing release are still read with helm get values, as before.

Fixes #481

[yxxhero]

Reviewed the change in depth against all call paths of template() — the fix is correct. Summary of the review, plus one small commit added on top.

Why it's correct

• template(isUpgrade) is called only from runHelm3() as d.template(!newInstall), and newInstall becomes true only after helm get manifest fails with release: not found while --allow-unreleased/--install is set. So gating the whole reuse block on isUpgrade means exactly "the release exists", which mirrors what helm upgrade --install does (ignore reuse flags for a new install).
• For an existing release (isUpgrade == true) the new condition is equivalent to the old one, so behavior there is unchanged.
• --dry-run=client/--dry-run=true paths remain safe: clusterAccessAllowed() is false there, so the block was already skipped.
• --revision > 0 on a missing release still errors out before template(), unchanged.
• The unreleased fake-helm mode fails helm get with Error: release: not found, which is what outputWithRichError surfaces and runHelm3 matches on — consistent with real helm. The only other get consumer (getChart in release.go) is not part of the upgrade flow.
• The dropped --reset-then-reuse-values version error for a missing release (helm < 3.14) is fine: the flag is ignored for a new install, so erroring about it would be inconsistent.

Commit added on top (08f8526): the rewrite moved isUpgrade out of shouldDefaultReusingValues, but no case exercised the two boundaries of that exact condition:

• existing release without any value flags → implicit default reuse must still fetch helm get values --all (guards shouldDefaultReusingValues against being dropped from the condition);
• new install without any value flags (bare --install, the most common new-release invocation) → must not fetch values.

Both cases are in TestUpgradeCommand_Execution_ReuseValues now. PR description case counts updated accordingly.