git: Fix deprecated subsection config syntax

Apparently marking subsections like `[section.subsection]` (toml-like)
is deprecated in the configuration. This commit fixes this by using the
recommended `[section "subsection"]` syntax.

See: https://git-scm.com/docs/git-config#_syntax

git/.config/git/config

@@ -41,14 +41,14 @@
   prompt        = false
 [difftool "difftastic"]
     cmd         = difft "$LOCAL" "$REMOTE"
-[color.diff]
+[color "diff"]
   meta          = "9"
   frag          = "magenta bold"
   commit        = "yellow bold"
   old           = "red bold"
   new           = "green bold"
   whitespace    = "red reverse"
-[color.diff-highlight]
+[color "diff-highlight"]
   oldNormal     = "red bold"
   oldHighlight  = "red bold 52"
   newNormal     = "green bold"