lakectl: switch tracked ref on local checkout even when the commit is unchanged

[ChrisJr404]

Closes #8361

Change Description

Background

lakectl local checkout --ref <other-branch> is supposed to switch the local directory to track a different ref. It works when the new ref points at a different commit, but when the requested ref resolves to the same commit the directory is already at, the tracked ref in .lakefs_ref.yaml is left untouched, so the directory keeps following the ref it was originally checked out from.

Bug Fix

1. Problem - checking out a ref that points at the current commit does not switch which ref the local directory tracks.
2. Root cause - localCheckout only rewrote the index when newHead != idx.AtHead, so a ref change with an unchanged head was ignored.
3. Solution - rewrite the index when the requested ref differs from the currently tracked ref as well, not only when the commit changes. When the head is unchanged there are no files to sync, so this just updates the tracked ref.

Testing Details

Added a unit test for the new checkoutSwitchesRef predicate covering same ref/same commit, different ref/same commit (the reported case), same ref/new commit, and different ref/different commit.

Breaking Change?

No. Existing checkouts that move to a different commit behave exactly as before.

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