merge(third_party/git): Merge squashed git subtree at v2.23.0

Merge commit '1b593e1ea4' as 'third_party/git'

third_party/git/mergetools/diffmerge

@@ -0,0 +1,18 @@
+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
+}
+
+exit_code_trustable () {
+	true
+}