Merge branch 'pb/mergetool-tool-help-fix' into maint

Fix 2.29 regression where "git mergetool --tool-help" fails to list
all the available tools.

* pb/mergetool-tool-help-fix:
  mergetool--lib: fix '--tool-help' to correctly show available tools

git-mergetool--lib.sh

@@ -46,9 +46,11 @@ show_tool_names () {
 		while read scriptname
 		do
 			setup_tool "$scriptname" 2>/dev/null
-			variants="$variants$(list_tool_variants)\n"
+			# We need an actual line feed here
+			variants="$variants
+$(list_tool_variants)"
 		done
-		variants="$(echo "$variants" | sort | uniq)"
+		variants="$(echo "$variants" | sort -u)"
 
 		for toolname in $variants
 		do

t/t7610-mergetool.sh

@@ -828,4 +828,15 @@ test_expect_success 'mergetool -Oorder-file is honored' '
 	test_cmp expect actual
 '
 
+test_expect_success 'mergetool --tool-help shows recognized tools' '
+	# Check a few known tools are correctly shown
+	git mergetool --tool-help >mergetools &&
+	grep vimdiff mergetools &&
+	grep vimdiff3 mergetools &&
+	grep gvimdiff2 mergetools &&
+	grep araxis mergetools &&
+	grep xxdiff mergetools &&
+	grep meld mergetools
+'
+
 test_done