Short-circuit git source updates for unchanged remote commits

[Lowerce]

This pull request introduces an optimization to the source update process for Git sources, ensuring that sources are not unnecessarily refreshed if the remote commit has not changed. It also adds supporting functionality and tests for reading the remote HEAD commit from a Git repository.

Optimization for Git source updates:

• The SourceAuthorityService now checks the remote HEAD commit for Git sources before performing a refresh. If the remote commit matches the locked commit, the refresh is skipped, improving efficiency.
• Added the helper method readLockedCommitSha to extract the commit SHA from a SourceRevision object.

New functionality for reading remote commits:

• Implemented readGitRemoteHeadCommit in SourceCheckoutService to retrieve the remote HEAD (or branch) commit SHA from a Git locator, with network retry logic.

Test coverage:

• Added a test to verify that updateSources skips the refresh if the remote commit is unchanged, and that prepareSourceCheckout is not called in this case.
• Added a test for readGitRemoteHeadCommit to confirm it correctly reads the remote HEAD commit from a Git repository.
• Updated test imports to include vi for mocking.