Fixed undetected working directory in root directory (the additional "dir.rfind" in the while condition stopped the loop *before* e.g. "C:" has been reached)
Fixed performance issue when executing IsValidGitDir on an UNC path (now we stop when reaching the root directory (provided by PathIsRoot), like "\\server\share")
Merge pull request #2306 from ulrichb/IsValidGitDir_UNC_path_bug

Shell Extensions: IsValidGitDir UNC path performance bug fix (resolves #2250)