Closes #865 - Add get_changed_items() utility for git-based selective deployment

[vipulb91]

Description

Briefly describe what this PR does and why.

Adds a new public utility function get_changed_items() to support git-based selective deployment of Fabric items.

Linked Issue (REQUIRED)

#865

Motivation

Closes #865. Users running CI/CD pipelines with fetchDepth: 2 (or similar shallow clones) want to deploy only the items that changed in a given commit/ref, rather than publishing all items every time.

## Approach (Option 2 — Decoupled) - Discussed in comment section with @shirasassoon

Rather than adding changed_only / git_compare_ref parameters directly to publish_all_items() and unpublish_all_orphan_items() (Option 1), this PR exposes the git-diff logic as a standalone utility function. This keeps the core publishing functions clean and allows users to compose the deployment logic themselves.

New API

from fabric_cicd import get_changed_items

changed = get_changed_items(repository_directory, git_compare_ref="HEAD~1")
# Returns a list of item names that were added/modified/renamed

if changed:
    publish_all_items(workspace, items_to_include=changed)

[vipulb91]

@microsoft-github-policy-service agree