fish_git_prompt: fix buggy rename parsing

Commit f86c9af4552 (fish_git_prompt: skip rename/copy source paths in
status parsing, 2026-05-12).  filters from "git-status --porcelain"
output all lines that don't look like a status entry.  This is
to filter out the rename source path.  But the filtering has false
positives, e.g. if a rename source path starts with "AA".  Let's only
skip lines immediately after each rename entry.  Also for copy (C)
though I haven't found a test case for that yet.