Try to get rid of SafeDispose() calls

[nulltoken]

SafeDispose() extension method takes care of ensuring the handle is valid before disposing it.

Let's try and get rid of this by moving the checks in SafeHandleBase.ReleaseHandle()

[nulltoken]

So, we can't get rid of SafeDispose() as is.

However, we can make it much simpler and makes its goal more obvious: Avoiding to have check if the handle is null before invoking Dispose().

Along the way I've found some places that could be simplified into a plain using () { } code block.

[dahlbyk]

Move declaration into using?

[nulltoken]

Done!

[nulltoken]

@Therzok One one your previous comment (that I can't seem to find anymore), was hinting at simplifying the code in WorkdirAndIndexToTree().

This is now done and the returned diff handle should be disposed (even on error) by the callers of BuildDiffList().

[Therzok]

I deleted it right after. I was sleepy and unsure of what I wrote. :D