Installer: Detect TortoiseGitPLink from TortoiseGit

TortoiseGit comes with a Git optimized version of TortoisePlink called TortoiseGitPLink. A new name was chosen so that TortoisePLink provides the exact same results as the TortoiseSVN version.

Main change to TortoiseGitPLink is that the "-batch" parameter which causes problems in TortoisePLink, but is required for vanilla plink, is ignored (see http://code.google.com/p/tortoisegit/issues/detail?id=1617).