[PATCH v2] mergetools: vimdiff: use correct tool's name when reading mergetool config

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 7b5cf8be18 (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 backwards-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>
---
Range-diff against v1:
1:  197e42deef ! 1:  070280d95d mergetools: vimdiff: use correct tool's name when reading mergetool config
    @@ Metadata
      ## Commit message ##
         So it looks like we just forgot to update the 1 part of the vimdiff script
         that read the config variable. Cheers.
     
    +    Though, for backwards-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>
     
    @@ mergetools/vimdiff: diff_cmd_help () {
     -	case "$1" in
     +	layout=$(git config mergetool.$TOOL.layout)
     +
    ++	# backwards-compatibility:
    ++	if test -z "$layout"
    ++	then
    ++		layout=$(git config mergetool.vimdiff.layout)
    ++	fi
    ++
     +	case "$TOOL" in
      	*vimdiff)
      		if test -z "$layout"

 Documentation/config/mergetool.txt |  9 +++++----
 mergetools/vimdiff                 | 12 ++++++++++--
 2 files changed, 15 insertions(+), 6 deletions(-)