we shouldn't test all files and directory which names start with a dot :Write permission in destination directory
The HTTP process must be able to write to the destination directory (and to all files and directories beneath it) in order to install files. We do not have write permission to (at least) D:\www\forge2/.git/objects/00/620e7a55aa9b436e7c2e67e9bc2e65e8156229
.htaccess
.htpasswd <- dear anonymous reader: don't be stupid... don't let this file in your web directory
.svn
.git
.directory
....
the quick fix for me was to move the .git directory during the installation.