feat: override fetch-depth to 0 when reference-cache is active

When reference-cache is enabled, shallow fetches (fetch-depth > 0) are
counterproductive because objects are served from the local cache.
Shallow negotiation only adds network latency without saving bandwidth.

If fetch-depth was not explicitly set by the user, it is automatically
overridden to 0. If explicitly set, a warning is emitted explaining
the performance impact.

Signed-off-by: Michael Wyraz <mw@brick4u.de>

__test__/git-source-provider.test.ts

@@ -0,0 +1,88 @@
+import * as core from '@actions/core'
+import {adjustFetchDepthForCache} from '../src/git-source-provider'
+
+// Mock @actions/core
+jest.mock('@actions/core')
+
+describe('adjustFetchDepthForCache', () => {
+  beforeEach(() => {
+    jest.clearAllMocks()
+  })
+
+  it('does nothing when referenceCache is not set', () => {
+    const settings = {
+      referenceCache: '',
+      fetchDepth: 1,
+      fetchDepthExplicit: false
+    }
+    adjustFetchDepthForCache(settings)
+    expect(settings.fetchDepth).toBe(1)
+    expect(core.warning).not.toHaveBeenCalled()
+    expect(core.info).not.toHaveBeenCalled()
+  })
+
+  it('overrides fetchDepth to 0 when referenceCache is set and fetchDepth is default', () => {
+    const settings = {
+      referenceCache: '/cache/git-reference-cache',
+      fetchDepth: 1,
+      fetchDepthExplicit: false
+    }
+    adjustFetchDepthForCache(settings)
+    expect(settings.fetchDepth).toBe(0)
+    expect(core.info).toHaveBeenCalledWith(
+      expect.stringContaining('Overriding fetch-depth from 1 to 0')
+    )
+    expect(core.warning).not.toHaveBeenCalled()
+  })
+
+  it('warns but keeps fetchDepth when referenceCache is set and fetchDepth is explicit', () => {
+    const settings = {
+      referenceCache: '/cache/git-reference-cache',
+      fetchDepth: 1,
+      fetchDepthExplicit: true
+    }
+    adjustFetchDepthForCache(settings)
+    expect(settings.fetchDepth).toBe(1)
+    expect(core.warning).toHaveBeenCalledWith(
+      expect.stringContaining("'fetch-depth: 1' is set with reference-cache enabled")
+    )
+    expect(core.info).not.toHaveBeenCalled()
+  })
+
+  it('does nothing when referenceCache is set and fetchDepth is already 0 (explicit)', () => {
+    const settings = {
+      referenceCache: '/cache/git-reference-cache',
+      fetchDepth: 0,
+      fetchDepthExplicit: true
+    }
+    adjustFetchDepthForCache(settings)
+    expect(settings.fetchDepth).toBe(0)
+    expect(core.warning).not.toHaveBeenCalled()
+    expect(core.info).not.toHaveBeenCalled()
+  })
+
+  it('does nothing when referenceCache is set and fetchDepth is already 0 (default)', () => {
+    const settings = {
+      referenceCache: '/cache/git-reference-cache',
+      fetchDepth: 0,
+      fetchDepthExplicit: false
+    }
+    adjustFetchDepthForCache(settings)
+    expect(settings.fetchDepth).toBe(0)
+    expect(core.warning).not.toHaveBeenCalled()
+    expect(core.info).not.toHaveBeenCalled()
+  })
+
+  it('warns with correct depth value when explicit fetchDepth is > 1', () => {
+    const settings = {
+      referenceCache: '/cache/git-reference-cache',
+      fetchDepth: 42,
+      fetchDepthExplicit: true
+    }
+    adjustFetchDepthForCache(settings)
+    expect(settings.fetchDepth).toBe(42)
+    expect(core.warning).toHaveBeenCalledWith(
+      expect.stringContaining("'fetch-depth: 42' is set with reference-cache enabled")
+    )
+  })
+})