installer: do configure some defaults in the system config

This part was unfortunately forgotten in c0fb1e5 (installer:
re-generate the system config instead of copying it, 2019-02-19)...

As a consequence, settings like the Git LFS configuration failed to be
set correctly.

This fixes git-for-windows/git#2140.

Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho

installer/install.iss

@@ -2402,6 +2402,13 @@ begin
             FileCopy(ExpandConstant('{tmp}\gitconfig.system'),AppDir+'\{#MINGW_BITNESS}\etc\gitconfig',True)
     end;
 
+    {
+        Configure some defaults in the system config
+    }
+
+    if not SetSystemConfigDefaults() then
+        LogError('Unable to set system config defaults');
+
     {
         Configure http.sslBackend according to the user's choice.
     }