Disable Shallow Fetch for AutoManagedVHD-Based Full Clones

[semihokur]

Context

AutoManagedVHDs provide the agent with a pre-mounted full Git clone of large repositories, allowing the checkout step to perform only a lightweight sync instead of cloning from scratch. However, some pipelines enable shallow fetch (fetchDepth > 0). Git cannot apply shallow semantics on top of an existing full clone. When a shallow fetch is attempted against a full clone, Git falls back to creating a brand-new shallow clone, which triggers an expensive full re-clone and eliminates the performance benefits of AutoManagedVHDs.

This PR ensures that when an AutoManagedVHD is detected, the Agent overrides shallow-fetch settings and performs a standard full fetch, preserving fast sync behavior.

This fix is also important for the broad, accelerated expansion of AutoManagedVHDs across Microsoft repositories, because it removes the need for teams to modify their pipeline YAML to override the fetchDepth parameter manually. AutoManagedVHD onboarding becomes seamless and configuration-free.

Description

Detects AutoManagedVHDs using the .autoManagedVhd marker file.

When detected, forces:
clean = false to retain the existing full clone, and
fetchDepth = 0 to disable shallow fetch.

Risk Assessment (Low / Medium / High)

The change only applies when a .autoManagedVhd file is present and does not affect normal checkout scenarios.

Unit Tests Added or Updated (Yes / No)

No

[sanjuyadav24]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[semihokur]

/azp run

[azure-pipelines]

Commenter does not have sufficient privileges for PR 5409 in repo microsoft/azure-pipelines-agent