Automatically disable git engine for Dependabot environments

[Copilot]

Summary

This PR enhances Nerdbank.GitVersioning to automatically disable the git engine when running under Dependabot, eliminating the need for manual configuration workarounds.

Problem

Dependabot operates on shallow clones, which causes Nerdbank.GitVersioning to fail since it requires access to git history. Previously, users needed to work around this limitation manually since Dependabot doesn't support custom environment variables. While a workaround existed (see dependabot/dependabot-core#4660), it required extra configuration that most users shouldn't need to manage.

Solution

This PR implements automatic detection of Dependabot environments:

• When the DEPENDABOT environment variable is set to "true" (case-insensitive) and the NBGV_GitEngine environment variable is not set, Nerdbank.GitVersioning automatically behaves as if NBGV_GitEngine=Disabled
• Users can still explicitly set NBGV_GitEngine to override this automatic behavior if needed
• The feature works seamlessly across all integration points: MSBuild tasks, the nbgv CLI tool, and cloud build commands

Implementation Details

1. Core Logic (GitContext.cs):

   • Added GetEffectiveGitEngine() public static method that centralizes engine selection logic
   • Checks for Dependabot environment via the DEPENDABOT environment variable
   • Parses NBGV_GitEngine values (LibGit2, Managed, Disabled) in one location
   • Always respects explicit NBGV_GitEngine settings (maintains user control)

2. Integration Points:

   • MSBuild (GetBuildVersion.cs): MSBuild GitEngine property takes highest precedence, followed by environment variables
   • CLI Tool (Program.cs): Updated nbgv commands to use the centralized logic
   • Cloud Command (CloudCommand.cs): Updated cloud build integration

3. Documentation (shallow-cloning-agents.md):

   • Updated Dependabot section to explain automatic behavior
   • Clarified that explicit NBGV_GitEngine setting overrides automatic detection
   • Noted that the previous workaround is no longer necessary for most users

Testing

• Added 22 comprehensive unit tests covering:
  • Default behavior with no environment variables
  • Automatic disabling when DEPENDABOT=true
  • Case-insensitive handling of DEPENDABOT values
  • Override behavior when NBGV_GitEngine is explicitly set
  • All valid NBGV_GitEngine values (LibGit2, Managed, Disabled)
• All 1,105 tests pass
• Manual testing confirms correct behavior across various environment configurations

Backward Compatibility

This change is fully backward compatible:

• Existing NBGV_GitEngine environment variable always takes precedence
• MSBuild GitEngine property continues to override environment variables
• Only affects Dependabot environments where NBGV_GitEngine is not set
• Invalid NBGV_GitEngine values are silently ignored (maintains existing behavior)
• No breaking changes to public APIs

Example Usage

# Dependabot automatically gets Engine.Disabled
export DEPENDABOT=true
# No NBGV_GitEngine set - engine is automatically disabled

# Users can still override if needed
export DEPENDABOT=true
export NBGV_GitEngine=LibGit2  # This takes precedence

Fixes #[issue-number]

Original prompt

Problem

Enhance Nerdbank.GitVersioning to automatically deactivate the git engine when running under Dependabot. Specifically:

• When the DEPENDABOT environment variable is set to true (case-insensitive), and the NBGV_GitEngine environment variable is not set, the tool should automatically behave as if NBGV_GitEngine is set to Disabled.
• This should effectively disable the git engine for Dependabot runs unless explicitly overridden by the user.

Tasks

1. Update the codebase so that on startup, if DEPENDABOT=true (case-insensitive) and NBGV_GitEngine is unset, the git engine is considered disabled.
2. Update the documentation at docfx\docs\shallow-cloning-agents.md to explain this behavior so users are aware this is now automatic for Dependabot.

Acceptance Criteria

• The git engine is disabled automatically in Dependabot environments unless overridden.
• Documentation clearly describes this logic and its rationale.

---

This PR implements the above logic and documentation update.

This pull request was created as a result of the following prompt from Copilot chat.

Problem

Enhance Nerdbank.GitVersioning to automatically deactivate the git engine when running under Dependabot. Specifically:

• When the DEPENDABOT environment variable is set to true (case-insensitive), and the NBGV_GitEngine environment variable is not set, the tool should automatically behave as if NBGV_GitEngine is set to Disabled.
• This should effectively disable the git engine for Dependabot runs unless explicitly overridden by the user.

Tasks

1. Update the codebase so that on startup, if DEPENDABOT=true (case-insensitive) and NBGV_GitEngine is unset, the git engine is considered disabled.
2. Update the documentation at docfx\docs\shallow-cloning-agents.md to explain this behavior so users are aware this is now automatic for Dependabot.

Acceptance Criteria

• The git engine is disabled automatically in Dependabot environments unless overridden.
• Documentation clearly describes this logic and its rationale.

---

This PR implements the above logic and documentation update.

---

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[AArnott]

@copilot You can find additional context on the problem being fixed here at dependabot/dependabot-core#4660