Reproduction steps

1. For a branch that has an existing reflog, call git_branch_move to rename the branch.
2. Notice the reflog was deleted and a new one created, with only a single line recording the rename.

Expected behavior

The reflog should be renamed and the rename record appended.

Actual behavior

The reflog is deleted and re-created.

Version of libgit2 (release number or SHA1)

Main (currently 1a94d97e5f5668d25d677471a81dba384267a428 at the time of this writing).

Operating system(s) tested

macOS 12.4