git: fix the config output formatting

When setting values using the `git config --set` command, git formats
the file a bit differently. This changes the output so it maps to that
format.

Differences:

* each `key = value` in a section is prefixed by a tab character
* the `=` between the key and the value is surrounded by spaces

PR #1069

tests/modules/programs/git/git.nix

@@ -15,8 +15,8 @@ let
     pkgs.substituteAll {
       src = path;
 
-      git_include_path =
-        pkgs.writeText "contents" (generators.toINI { } gitInclude);
+      git_include_path = pkgs.writeText "contents"
+        (builtins.readFile ./git-expected-include.conf);
     };
 
 in {