fish_git_prompt: skip rename/copy source paths in status parsing

When git status --porcelain -z reports renames or copies, it outputs
the source filename as a separate NUL-delimited field after the status
line. This extra entry was counted as an additional change, inflating
staged/dirty counts when the source filename started with [ACDMRTU].

Filter split results to entries matching a valid two-char status code
prefix, which excludes the bare source filenames.

Fixes #11296

Part of #12754