githooks: discuss Git operations in foreign repositories

Hook authors are periodically caught off-guard by difficult-to-diagnose
errors when their hook invokes Git commands in a repository other than
the local one. In particular, Git environment variables, such as GIT_DIR
and GIT_WORK_TREE, which reference the local repository cause the Git
commands to operate on the local repository rather than on the
repository which the author intended. This is true whether the
environment variables have been set manually by the user or
automatically by Git itself. The same problem crops up when a hook
invokes Git commands in a different worktree of the same repository, as
well.

Recommended best-practice[1,2,3,4,5,6] for avoiding this problem is for
the hook to ensure that Git variables are unset before invoking Git
commands in foreign repositories or other worktrees:

    unset $(git rev-parse --local-env-vars)

However, this advice is not documented anywhere. Rectify this
shortcoming by mentioning it in githooks.txt documentation.

[1]: https://lore.kernel.org/git/YFuHd1MMlJAvtdzb@coredump.intra.peff.net/
[2]: https://lore.kernel.org/git/20200228190218.GC1408759@coredump.intra.peff.net/
[3]: https://lore.kernel.org/git/20190516221702.GA11784@sigill.intra.peff.net/
[4]: https://lore.kernel.org/git/20190422162127.GC9680@sigill.intra.peff.net/
[5]: https://lore.kernel.org/git/20180716183942.GB22298@sigill.intra.peff.net/
[6]: https://lore.kernel.org/git/20150203163235.GA9325@peff.net/

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Acked-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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