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-auth-helper.test.ts

@@ -1159,6 +1159,7 @@ async function setup(testName: string): Promise<void> {
     sparseCheckout: [],
     sparseCheckoutConeMode: true,
     fetchDepth: 1,
+    fetchDepthExplicit: false,
     fetchTags: false,
     showProgress: true,
     lfs: false,