Re: [PATCH] mergetool: new config guiDefault supports auto-toggling gui by DISPLAY

> From: Tao Klerks <tao@klerks.biz>
>
> 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.
>
> Introduce new configuration options, difftool.guiDefault and
> mergetool.guiDefault, supporting a special value "auto" which causes the
> corresponding tool or guitool to be selected depending on the presence of a
> non-empty DISPLAY value. Also support "true" to say "default to the guitool
> (unless --no-gui is passed on the commandline)", and "false" as the previous
> default behavior when these new configuration options are not specified.
>
> Signed-off-by: Tao Klerks <tao@klerks.biz>
> ---
>     RFC: mergetool: new config guiDefault supports auto-toggling gui by
>     DISPLAY