Bigger buffer for utf-8 parsing in win32

This changes the size of path buffers to be `MAX_PATH+1` 16-bit characters, or `MAX_PATH*4+1` 8-bit characters, to allow for inflation when converting to/from utf-8.

Fixes #1752.


## To be done
- [x] Increase utf-8 buffer sizes
- [x] Provide a proper typedef for path buffers
- [x] Make conversion routines more consistent
- [x] Test status
- [x] Test checkout