installer: do not leave empty directories behind

When uninstalling Git for Windows, a C:\Program Files\Git\mingw64 folder
was left behind, with no files in it, leading to problems for subsequent
installations (that would ask whether the user wants to overwrite any
existing installation).

First noticed when uninstalling v2.10.1, after installing v2.10.0 with
admin privileges to %PROGRAMFILES%/Git and upgrading to v2.10.1 on
Windows 7 x64, that the uninstaller did not clean up %PROGRAMFILES%/Git
nor %PROGRAMFILES%/Git/mingw64.

This fixes git-for-windows/git#909

[JES: touched up commit message; SOB not needed, as I suggested the
patch to begin with]

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

Author: Samuel Lijin

installer/install.iss

@@ -236,6 +236,8 @@ Type: dirifempty; Name: {app}\etc
 ; Delete recorded install options
 Type: files; Name: {app}\etc\install-options.txt
 Type: dirifempty; Name: {app}\etc
+Type: dirifempty; Name: {app}\{#MINGW_BITNESS}
+Type: dirifempty; Name: {app}
 
 [Code]
 #include "helpers.inc.iss"