Better error message for corrupt .git/index

A user received this error when they had a bad index.

Information {"Message":"System.ArgumentOutOfRangeException: Non-negative number required.\r\nParameter name: dstOffset\r\n at System.Buffer.BlockCopy(Array src, Int32 srcOffset, Array dst, Int32 dstOffset, Int32 count)\r\n at GVFS.Virtualization.Projection.GitIndexProjection.GitIndexParser.ReadPath(GitIndexEntry indexEntryData, Int32 replaceIndex, Int32 byteCount)\r\n at GVFS.Virtualization.Projection.GitIndexProjection.GitIndexParser.ParseIndex(ITracer tracer, Stream indexStream, GitIndexEntry resuableParsedIndexEntry, Func2 entryAction)\r\n at GVFS.Virtualization.Projection.GitIndexProjection.GitIndexParser.RebuildProjection(ITracer tracer, Stream indexStream)\r\n at GVFS.Virtualization.Projection.GitIndexProjection.BuildProjectionFromPath(ITracer tracer, String indexPath)\r\n at GVFS.RepairJobs.RepairJob.TryParseIndex(String path, List1 messages) in

Could it be more beneficial to run 'git ls-files' in and print out the error?
Can we handle this better?

@kewillford @wilbaker @jamill @derrickstolee for everyone's thoughts.

Start with GitIndexProjection.GitIndexParser.ReadPath and ParseIndex in the stack trace, then follow the call into RepairJob.TryParseIndex. Reproduce the corrupt-index case and inspect the existing diagnostic path; done means the failure produces a useful user-facing error rather than the reported ArgumentOutOfRangeException.