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>
fixup! mergetool--lib: add functions for finding available tools

Signed-off-by: John Keeping <john@keeping.me.uk>
fixup!  doc: generate a list of valid merge tools

Signed-off-by: John Keeping <john@keeping.me.uk>
mergetool--lib: list user configured tools in '--tool-help'

Signed-off-by: John Keeping <john@keeping.me.uk>