fix(git): preserve merge commits in log output

[fengjikui]

Fixes #2305.

Summary

• stop adding --no-merges to default rtk git log invocations
• preserve merge commits for plain log output and formatted output such as --oneline
• factor log command setup so the default plan is covered by regression tests

Why

rtk git log previously injected --no-merges whenever the user did not pass an explicit count, which made merge commits invisible in common verification commands like rtk git log --oneline. Merge commits can be HEAD, so hiding them changes normal git log semantics and breaks merge/PR verification.

Validation

• cargo fmt --check
• cargo test cmds::git::git::tests::test_configure_log_command_preserves_merge_commits
• cargo test cmds::git::git::tests
• cargo clippy --all-targets -- -D warnings
• cargo test (2147 passed, 8 ignored)
• CLI smoke in a temp repo with a real merge commit: raw git log --oneline and rtk git log --oneline both listed the merge commit first.