mergetools

The /mergetools/vimdiff script, which handles both vimdiff, nvimdiff
and gvimdiff mergetools (the latter 2 simply source the vimdiff script), has a
function merge_cmd() which read the layout variable from git config, and it
would always read the value of mergetool.**vimdiff**.layout, instead of the
mergetool being currently used (vimdiff or nvimdiff or gvimdiff).

It looks like in 7b5cf8be (vimdiff: add tool documentation, 2022-03-30),
we explained the current behavior in Documentation/config/mergetool.txt:

```
mergetool.vimdiff.layout::
	The vimdiff backend uses this variable to control how its split
	windows look like. Applies even if you are using Neovim (`nvim`) or
	gVim (`gvim`) as the merge tool. See BACKEND SPECIFIC HINTS section
```

which makes sense why it's explained this way - the vimdiff backend is used by
gvim and nvim. But the mergetool's configuration should be separate for each tool,
and indeed that's confirmed in same commit at Documentation/mergetools/vimdiff.txt:

```
Variants

Instead of `--tool=vimdiff`, you can also use one of these other variants:
  * `--tool=gvimdiff`, to open gVim instead of Vim.
  * `--tool=nvimdiff`, to open Neovim instead of Vim.

When using these variants, in order to specify a custom layout you will have to
set configuration variables `mergetool.gvimdiff.layout` and
`mergetool.nvimdiff.layout` instead of `mergetool.vimdiff.layout`
```

So it looks like we just forgot to update the 1 part of the vimdiff script
that read the config variable. Cheers.

Though, for backward compatibility, I've kept the mergetool.vimdiff
fallback, so that people who unknowingly relied on it, won't have their
setup broken now.

Signed-off-by: Kipras Melnikovas <kipras@kipras.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>