git status should not fail when it is unable to open cache file

[jamill]

In microsoft/VFSForGit#663, a functional test failed to run status because git encountered an error (access denied) when reading from a status cache file.

Reading status from a cache file is a best effort operation. If the file does not exist, we fall back to computing a fresh status. The intention is that failing to read from the cache file is also a best effort, and we should not terminate if we are unable to read an existing file.

We should be more resilient to these types of errors, and fallback to computing full status gracefully rather than terminating. The issue here is the utility xopen function used by status cache deserialization logic will terminate if it encounters an issue reading the file.