49128 (github #76): vcs_info-examples: optimize +vi-git-untracked()

Speed up the prompt on large and/or deep working directories by
stopping grep(1) as soon as it finds a single match, with `-q`.
Also, correct the regexp by adding a ^ anchor and increase its
specificity by accounting for the space in Porcelain Format v1.

Previously, +vi-git-untracked() waited for grep(1) to find all
matches of untracked files, redirecting them away to /dev/null,
before finally concluding that untracked files do indeed exist.

With this patch, I see 4x speedup on a large Git-enabled $HOME:

    $ time (git status --porcelain | wc -l)
    212
    0.01s user 0.02s system 0% cpu 9.021 total

    $ time (git status --porcelain | grep '??' &>/dev/null)
    0.01s user 0.02s system 0% cpu 12.294 total

    $ time (git status --porcelain | grep -q '^?? ' 2>/dev/null)
    0.01s user 0.01s system 0% cpu 3.097 total

Note that `-q` for grep(1) is in POSIX (IEEE Std 1003.1-2017):
https://pubs.opengroup.org/onlinepubs/9699919799/utilities/grep.html

See discussion https://www.zsh.org/mla/workers/2021/msg01354.html

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