From: Lin Sun <lin.sun@zoom.us>

Make the mergetool used with "meld" backend behave similarly to "vimdiff" by
telling it to auto-merge non-conflicting parts and highlight the conflicting
parts when `mergetool.meld.useAutoMerge` is configured with `true`, or `auto`
for detecting the `--auto-merge` option automatically.

Helped-by: Đoàn Trần Công Danh <congdanhqx@gmail.com>
Helped-by: Junio C Hamano <gitster@pobox.com>
Helped-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Lin Sun <lin.sun@zoom.us>

Submitted-As: https://lore.kernel.org/git/pull.781.v9.git.git.1594002423685.gitgitgadget@gmail.com
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