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

modules/programs/git.nix

@@ -22,7 +22,7 @@ let
   # generation for multiple ini values
   mkKeyValue = k: v:
     let mkKeyValue = generators.mkKeyValueDefault { } " = " k;
-    in concatStringsSep "\n" (map mkKeyValue (toList v));
+    in concatStringsSep "\n" (map (kv: "	" + mkKeyValue kv) (toList v));
 
   # converts { a.b.c = 5; } to { "a.b".c = 5; } for toINI
   gitFlattenAttrs = let

tests/modules/programs/git/git-expected-include.conf

@@ -0,0 +1,3 @@
+[user]
+	email = user@example.org
+	name = John Doe

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 {