doc: generate a list of valid merge tools

Use the show_tool_names() function to build lists of all
the built-in tools supported by difftool and mergetool.
This frees us from needing to update the documentation
whenever a new tool is added.

Signed-off-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-mergetool--lib.sh

@@ -1,6 +1,7 @@
 #!/bin/sh
 # git-mergetool--lib is a library for common merge tool functions
-MERGE_TOOLS_DIR=$(git --exec-path)/mergetools
+
+: ${MERGE_TOOLS_DIR=$(git --exec-path)/mergetools}
 
 mode_ok () {
 	if diff_mode