_omz_git_prompt_status was emitting "regex matching error: illegal byte
sequence" when the git branch name contained non-ASCII characters (e.g.
Chinese). The zsh =~ operator uses locale-aware POSIX ERE, so character
classes like [^ ]+ reject multibyte sequences unless LC_ALL=C is set.
Fix by setting LC_ALL=C around all regex operations in the function and
restoring it afterwards.
Fixes#13330
