mergetool: new config guiDefault supports auto-toggling gui by DISPLAY

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
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>
Acked-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config/difftool.txt

@@ -34,3 +34,10 @@ See the `--trust-exit-code` option in linkgit:git-difftool[1] for more details.
 
 difftool.prompt::
 	Prompt before each invocation of the diff tool.
+
+difftool.guiDefault::
+	Set `true` to use the `diff.guitool` by default (equivalent to specifying
+	the `--gui` argument), or `auto` to select `diff.guitool` or `diff.tool`
+	depending on the presence of a `DISPLAY` environment variable value. The
+	default is `false`, where the `--gui` argument must be provided
+	explicitly for the `diff.guitool` to be used.

Documentation/config/mergetool.txt

@@ -85,3 +85,10 @@ mergetool.writeToTemp::
 
 mergetool.prompt::
 	Prompt before each invocation of the merge resolution program.
+
+mergetool.guiDefault::
+	Set `true` to use the `merge.guitool` by default (equivalent to
+	specifying the `--gui` argument), or `auto` to select `merge.guitool`
+	or `merge.tool` depending on the presence of a `DISPLAY` environment
+	variable value. The default is `false`, where the `--gui` argument
+	must be provided explicitly for the `merge.guitool` to be used.

Documentation/git-difftool.txt

@@ -97,10 +97,12 @@ instead.  `--no-symlinks` is the default on Windows.
 --[no-]gui::
 	When 'git-difftool' is invoked with the `-g` or `--gui` option
 	the default diff tool will be read from the configured
-	`diff.guitool` variable instead of `diff.tool`. The `--no-gui`
-	option can be used to override this setting. If `diff.guitool`
-	is not set, we will fallback in the order of `merge.guitool`,
-	`diff.tool`, `merge.tool` until a tool is found.
+	`diff.guitool` variable instead of `diff.tool`. This may be
+	selected automatically using the configuration variable
+	`difftool.guiDefault`. The `--no-gui` option can be used to
+	override these settings. If `diff.guitool` is not set, we will
+	fallback in the order of `merge.guitool`, `diff.tool`,
+	`merge.tool` until a tool is found.
 
 --[no-]trust-exit-code::
 	'git-difftool' invokes a diff tool individually on each file.

Documentation/git-mergetool.txt

@@ -85,12 +85,13 @@ success of the resolution after the custom tool has exited.
 	the default merge tool will be read from the configured
 	`merge.guitool` variable instead of `merge.tool`. If
 	`merge.guitool` is not set, we will fallback to the tool
-	configured under `merge.tool`.
+	configured under `merge.tool`. This may be autoselected using
+	the configuration variable `mergetool.guiDefault`.
 
 --no-gui::
-	This overrides a previous `-g` or `--gui` setting and reads the
-	default merge tool will be read from the configured `merge.tool`
-	variable.
+	This overrides a previous `-g` or `--gui` setting or
+	`mergetool.guiDefault` configuration and reads the default merge
+	tool from the configured `merge.tool` variable.
 
 -O<orderfile>::
 	Process files in the order specified in the