mergetools/diffmerge: support DiffMerge as a git mergetool

DiffMerge is a non-free (but gratis) tool that supports OS X, Windows and Linux.

    See http://www.sourcegear.com/diffmerge/

DiffMerge includes a script `/usr/bin/diffmerge` that can be used to launch the
graphical compare tool.

This change adds mergetool support for DiffMerge and adds 'diffmerge' as an
option to the mergetool help.

Signed-off-by: Stefan Saasen <ssaasen@atlassian.com>
Acked-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>

mergetools/diffmerge

@@ -0,0 +1,15 @@
+diff_cmd () {
+	"$merge_tool_path" "$LOCAL" "$REMOTE" >/dev/null 2>&1
+}
+
+merge_cmd () {
+	if $base_present
+	then
+		"$merge_tool_path" --merge --result="$MERGED" \
+			"$LOCAL" "$BASE" "$REMOTE"
+	else
+		"$merge_tool_path" --merge \
+			--result="$MERGED" "$LOCAL" "$REMOTE"
+	fi
+	status=$?
+}