fix: Declare merged_git_metadata_settings before usage

[AbhiPrasad]

resolves #8

AI Summary

Fix: One-line initialization at logger.py:1642 — merged_git_metadata_settings = None

Bug: When repo_info is passed to braintrust.init(), the code skips the else block where merged_git_metadata_settings gets assigned (line 1646), but later references it unconditionally on line 1660. This causes an UnboundLocalError whenever base_experiment_id and base_experiment are both None (the default).

Fix behavior: Initializing to None before the if/else ensures the variable is always defined. Since None is falsy, the elif on line 1660 short-circuits correctly — skipping ancestor commit collection, which is the right behavior when the user explicitly provides repo_info.

Test: Added test_init_with_repo_info_does_not_raise in test_logger.py that calls braintrust.init(project=..., repo_info=RepoInfo(...)) and forces compute_metadata() to execute. Verified it fails with UnboundLocalError without the fix and passes with it.