mergetool: allow auto-merge for meld to follow the vim-diff behavior

mergetool: allow auto-merge for meld to follow the vim-diff behavior

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 <redacted>
Helped-by: David Aguilar <redacted>
Signed-off-by: Lin Sun <redacted>
Signed-off-by: Junio C Hamano <redacted>