Merge branch 'dl/mergetool-gui-option'

"git mergetool" learned to take the "--[no-]gui" option, just like
"git difftool" does.

* dl/mergetool-gui-option:
  doc: document diff/merge.guitool config keys
  completion: support `git mergetool --[no-]gui`
  mergetool: accept -g/--[no-]gui as arguments

Documentation/diff-config.txt

@@ -177,6 +177,14 @@ diff.tool::
 	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/git-mergetool.txt

@@ -79,6 +79,17 @@ success of the resolution after the custom tool has exited.
 	Prompt before each invocation of the merge resolution program
 	to give the user a chance to skip the path.
 
+-g::
+--gui::
+	When 'git-mergetool' is invoked with the `-g` or `--gui` option
+	the default merge tool will be read from the configured
+	`merge.guitool` variable instead of `merge.tool`.
+
+--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.
+
 -O<orderfile>::
 	Process files in the order specified in the
 	<orderfile>, which has one shell glob pattern per line.

Documentation/merge-config.txt

@@ -63,6 +63,12 @@ merge.tool::
 	Any other value is treated as a custom merge tool and requires
 	that a corresponding mergetool.<tool>.cmd variable is defined.
 
+merge.guitool::
+	Controls which merge tool is used by linkgit:git-mergetool[1] when the
+	-g/--gui flag is specified. The list below shows the valid built-in values.
+	Any other value is treated as a custom merge tool and requires that a
+	corresponding mergetool.<guitool>.cmd variable is defined.
+
 include::mergetools-merge.txt[]
 
 merge.verbosity::