difftool docs: de-duplicate configuration sections

Include the "config/difftool.txt" file in "git-difftool.txt", and move
the relevant part of git-difftool(1) configuration from
"config/diff.txt" to config/difftool.txt".

Doing this is slightly odd, as we usually discuss configuration in
alphabetical order, but by doing it we're able to include the full set
of configuration used by git-difftool(1) (and only that configuration)
in its own documentation.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Reviewed-by: Matheus Tavares <matheus.bernardino@usp.br>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config/diff.txt

@@ -178,21 +178,6 @@ diff.<driver>.cachetextconv::
 	Set this option to true to make the diff driver cache the text
 	conversion outputs.  See linkgit:gitattributes[5] for details.
 
-diff.tool::
-	Controls which diff tool is used by linkgit:git-difftool[1].
-	This variable overrides the value configured in `merge.tool`.
-	The list below shows the valid built-in values.
-	Any other value is treated as a custom diff tool and requires
-	that a corresponding difftool.<tool>.cmd variable is defined.
-
-diff.guitool::
-	Controls which diff tool is used by linkgit:git-difftool[1] when
-	the -g/--gui flag is specified. This variable overrides the value
-	configured in `merge.guitool`. The list below shows the valid
-	built-in values. Any other value is treated as a custom diff tool
-	and requires that a corresponding difftool.<guitool>.cmd variable
-	is defined.
-
 include::../mergetools-diff.txt[]
 
 diff.indentHeuristic::

Documentation/config/difftool.txt

@@ -1,6 +1,17 @@
-difftool.<tool>.path::
+diff.tool::
-	Override the path for the given tool.  This is useful in case
+	Controls which diff tool is used by linkgit:git-difftool[1].
-	your tool is not in the PATH.
+	This variable overrides the value configured in `merge.tool`.
+	The list below shows the valid built-in values.
+	Any other value is treated as a custom diff tool and requires
+	that a corresponding difftool.<tool>.cmd variable is defined.
+
+diff.guitool::
+	Controls which diff tool is used by linkgit:git-difftool[1] when
+	the -g/--gui flag is specified. This variable overrides the value
+	configured in `merge.guitool`. The list below shows the valid
+	built-in values. Any other value is treated as a custom diff tool
+	and requires that a corresponding difftool.<guitool>.cmd variable
+	is defined.
 
 difftool.<tool>.cmd::
 	Specify the command to invoke the specified diff tool.
@@ -9,6 +20,17 @@ difftool.<tool>.cmd::
 	file containing the contents of the diff pre-image and 'REMOTE'
 	is set to the name of the temporary file containing the contents
 	of the diff post-image.
++
+See the `--tool=<tool>` option in linkgit:git-difftool[1] for more details.
+
+difftool.<tool>.path::
+	Override the path for the given tool.  This is useful in case
+	your tool is not in the PATH.
+
+difftool.trustExitCode::
+	Exit difftool if the invoked diff tool returns a non-zero exit status.
++
+See the `--trust-exit-code` option in linkgit:git-difftool[1] for more details.
 
 difftool.prompt::
 	Prompt before each invocation of the diff tool.

Documentation/git-difftool.txt

@@ -113,33 +113,14 @@ instead.  `--no-symlinks` is the default on Windows.
 
 See linkgit:git-diff[1] for the full list of supported options.
 
-CONFIG VARIABLES
+CONFIGURATION
-----------------
+-------------
 'git difftool' falls back to 'git mergetool' config variables when the
 difftool equivalents have not been defined.
 
-diff.tool::
+include::includes/cmd-config-section-rest.txt[]
-	The default diff tool to use.
 
-diff.guitool::
+include::config/difftool.txt[]
-	The default diff tool to use when `--gui` is specified.
-
-difftool.<tool>.path::
-	Override the path for the given tool.  This is useful in case
-	your tool is not in the PATH.
-
-difftool.<tool>.cmd::
-	Specify the command to invoke the specified diff tool.
-+
-See the `--tool=<tool>` option above for more details.
-
-difftool.prompt::
-	Prompt before each invocation of the diff tool.
-
-difftool.trustExitCode::
-	Exit difftool if the invoked diff tool returns a non-zero exit status.
-+
-See the `--trust-exit-code` option above for more details.
 
 SEE ALSO
 --------