doc: consolidate --rerere-autoupdate description

The `--rerere-autoupdate` option is shared across 5 commands, and
are described the same way because it works exactly the same way in
these commands.

Create a separate file and include it from the help pages for these
commands, so that we can improve the description at one place to
improve all of them at once, and keep them in sync.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

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