zshrc: fix problems in vcs_info wrt older git versions

zshrc: fix problems in vcs_info wrt older git versions

When rebases conflict and create a directory .dotest/, the branch name
ended up empty and that caused the git backend to abort collecting data
and return an error code.

That caused vcs_info to set up everything as if no version control
system controlled the directory.

Reported by Timo Boettcher, Thanks!