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

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

Commit 83bbf9b92e (mergetool--lib: improve support for vimdiff-style tool
variants, 2020-07-29) introduced a regression in the output of `git mergetool
--tool-help` and `git difftool --tool-help` [1].

In function 'show_tool_names' in git-mergetool--lib.sh, we loop over the
supported mergetools and their variants and accumulate them in the variable
'variants', separating them with a literal '\n'.

The code then uses 'echo $variants' to turn these '\n' into newlines, but this
behaviour is not portable, it just happens to work in some shells, like
dash(1)'s 'echo' builtin.

For shells in which 'echo' does not turn '\n' into newlines, the end
result is that the only tools that are shown are the existing variants
(except the last variant alphabetically), since the variants are
separated by actual newlines in '$variants' because of the several
'echo' calls in mergetools/{bc,vimdiff}::list_tool_variants.

Fix this bug by embedding an actual line feed into `variants` in
show_tool_names(). While at it, replace `sort | uniq` by `sort -u`.

To prevent future regressions, add a simple test that checks that a few
known tools are correctly shown (let's avoid counting the total number
of tools to lessen the maintenance burden when new tools are added or if
'--tool-help' learns additional logic, like hiding tools depending on
the current platform).

[1] https://lore.kernel.org/git/CADtb9DyozjgAsdFYL8fFBEWmq7iz4=prZYVUdH9W-J5CKVS4OA@mail.gmail.com/

Reported-by: Philippe Blain <levraiphilippeblain@gmail.com>
Based-on-patch-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Philippe Blain <levraiphilippeblain@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>