git plugin should handle a situation when cache .git/config file gets corrupted

due to BSP-5470 and BSP-4372

the git plugin should recognize 'Unknown repository format "null"; expected "0"' exception and gently handle it. For example it might clean the cache directory - that action is expected to fix the problem, but might be too expensive on the long run. Otherwise it might be good idea to detect and fix corrupted .git/config files - as it seems in some circumstances that file is empty, or at least it doesn't contain "repositoryformatversion = 0"

probably the issue occurs on Windows only