checkout.txt: note about losing staged changes with --merge

If you have staged changes in path A and perform 'checkout
--merge' (which could result in conflicts in a totally unrelated path
B), changes in A will be gone. Which is unexpected. We are supposed
to keep all changes, or kick and scream otherwise.

This is the result of how --merge is implemented, from the very first
day in 1be0659efc (checkout: merge local modifications while switching
branches., 2006-01-12):

1. a merge is done, unmerged entries are collected
2. a hard switch to a new branch is done, then unmerged entries added
   back

There is no trivial fix for this. Going with 3-way merge one file at a
time loses rename detection. Going with 3-way merge by trees requires
teaching the algorithm to pick up staged changes. And even if we detect
staged changes with --merge and abort for safety, an option to continue
--merge is very weird. Such an option would keep worktree changes, but
drop staged changes.

Because the problem has been with us since the introduction of --merge
and everybody has been pretty happy (except Phillip, who found this
problem), I'll just take a note here to acknowledge it and wait for
merge wizards to come in and work their magic. There may be a way
forward [1].

[1] CABPp-BFoL_U=bzON4SEMaQSKU2TKwnOgNqjt5MUaOejTKGUJxw@mail.gmail.com

Reported-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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