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When no merge.tool or diff.tool is configured or manually selected, the selection of a default tool is sensitive to the DISPLAY variable; in a GUI session a gui-specific tool will be proposed if found, and otherwise a terminal-based one. This "GUI-optimizing" behavior is important because a GUI can make a huge difference to a user's ability to understand and correctly complete a non-trivial conflicting merge. Some time ago the merge.guitool and diff.guitool config options were introduced to enable users to configure both a GUI tool, and a non-GUI tool (with fallback if no GUI tool configured), in the same environment. Unfortunately, the --gui argument introduced to support the selection of the guitool is still explicit. When using configured tools, there is no equivalent of the no-tool-configured "propose a GUI tool if we are in a GUI environment" behavior. As proposed in <xmqqmtb8jsej.fsf@gitster.g>, introduce new configuration opt...
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