installer: fix privileges (again)

Turns out that we had *plenty* of problems with first switching
PrivilegesRequired to 'poweruser' and then to 'lowest': now upgrades
from 2.6.2 to 2.6.3 cannot write to C:\Program Files anymore.

Let's just revert the change and hope that this fixes the upgrade
problem.

In particular, this commit reverts 1f3e1f7 (installer: Change installer
privileges to 'lowest', 2015-10-20) and ec49475 (installer: clarify
required privileges, 2015-10-14).

The deeper explanation is that InnoSetup's `none` and `lowest` settings
are different in behavior, no matter what the release notes made us
believe. The `none` setting will actually respawn an elevated installer
if the current user is in the admin or power user group: see
https://github.com/jrsoftware/issrc/blob/841a86c32/Projects/Main.pas#L2939-L2940
and
https://github.com/jrsoftware/issrc/blob/ea65d280c/Projects/SpawnServer.pas#L217.

Therefore, the `none` setting is actually exactly what we want: users
who have credentials to write to C:\Program Files are presented with an
elevated installer that let's them do precisely that, users without
those rights will be presented with an installer that let's them install
Git into AppData in their home directory.

This fixes git-for-windows/git#523,
git-for-windows/git#526 and
git-for-windows/git#528.

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

installer/install.iss

@@ -55,7 +55,11 @@ DefaultGroupName={#APP_NAME}
 DisableProgramGroupPage=auto
 DisableReadyPage=yes
 InfoBeforeFile={#SourcePath}\gpl-2.0.rtf
+#ifdef OUTPUT_TO_TEMP
 PrivilegesRequired=lowest
+#else
+PrivilegesRequired=none
+#endif
 UninstallDisplayIcon={app}\{#MINGW_BITNESS}\share\git\git-for-windows.ico
 #ifndef COMPILE_FROM_IDE
 #if Pos('-',APP_VERSION)>0