Fix failure in win32_find_git_in_registry()

The buffer size 0 was definitely not enough so it failed
Fix failure in win32_find_git_in_registry() when UAC is turned on

Demand read only access to registry key instead of full access.
This might happen in Windows Vista and later.
Merge pull request #1857 from linquize/find-git-path-win32

Fix failure in win32 git path guessing