Merge branch 'fr/vimdiff-layout'

Reimplement "vimdiff[123]" mergetool drivers with a more generic
layout mechanism.

* fr/vimdiff-layout:
  mergetools: add description to all diff/merge tools
  vimdiff: add tool documentation
  vimdiff: integrate layout tests in the unit tests framework ('t' folder)
  vimdiff: new implementation with layout support

Documentation/config/mergetool.txt

@@ -45,6 +45,15 @@ mergetool.meld.useAutoMerge::
 	value of `false` avoids using `--auto-merge` altogether, and is the
 	default value.
 
+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
+ifndef::git-mergetool[]
+	in linkgit:git-mergetool[1].
+endif::[]
+	for details.
+
 mergetool.hideResolved::
 	During a merge Git will automatically resolve as many conflicts as
 	possible and write the 'MERGED' file containing conflict markers around